If QEMU shuts down while we're in the middle of processing a
monitor command, the monitor will be freed, and upon cleaning
up we attempt to do  qemuMonitorUnlock(priv->mon) when priv->mon
is NULL.

To address this we introduce proper reference counting into
the qemuMonitorPtr object, and hold an extra reference whenever
executing a command.

* src/qemu/qemu_driver.c: Hold a reference on the monitor while
  executing commands, and only NULL-ify the priv->mon field when
  the last reference is released
* src/qemu/qemu_monitor.h, src/qemu/qemu_monitor.c: Add reference
  counting to handle safe deletion of monitor objects

The QEMU monitor open method would not take a reference on
the virDomainObjPtr until it had successfully opened the
monitor. The cleanup code upon failure to open though would
call qemuMonitorClose() which would in turn decrement the
reference count. This caused the virDoaminObjPtr to be mistakenly
freed and then the whole driver crashes

* src/qemu/qemu_monitor.c: Fix reference counting in
  qemuMonitorOpen

* src/qemu/qemu_monitor.c: add error check for qemuMonitorOpenXXX
  returned file descriptor

* src/qemu/qemu_monitor.c src/qemu/qemu_monitor_text.c: cast size_t to
  int when passing to '%d'

Change the QEMU monitor file handle watch to poll for both
read & write events, as well as EOF. All I/O to/from the
QEMU monitor FD is now done in the event callback thread.

When the QEMU driver needs to send a command, it puts the
data to be sent into a qemuMonitorMessagePtr object instance,
queues it for dispatch, and then goes to sleep on a condition
variable. The event thread sends all the data, and then waits
for the reply to arrive, putting the response / error data
back into the qemuMonitorMessagePtr and notifying the condition
variable.

There is a temporary hack in the disk passphrase callback to
avoid acquiring the domain lock.  This avoids a deadlock in
the command processing, since the domain lock is still held
when running monitor commands. The next commit will remove
the locking when running commands & thus allow re-introduction
of locking the disk passphrase callback

* src/qemu/qemu_driver.c: Temporarily don't acquire lock in
  disk passphrase callback. To be reverted in next commit
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h: Remove
  raw I/O functions, and a generic qemuMonitorSend() for
  invoking a command
* src/qemu/qemu_monitor_text.c, src/qemu/qemu_monitor_text.h:
  Remove all low level I/O, and use the new qemuMonitorSend()
  API. Provide a qemuMonitorTextIOProcess() method for detecting
  command/reply/prompt boundaries in the monitor data stream

In preparation of the monitor I/O process becoming fully asynchronous,
it is neccessary to ensure all access to internals of the qemuMonitorPtr
object is protected by a mutex lock.

* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h: Add mutex for locking
  monitor.
* src/qemu/qemu_driver.c: Add locking around all monitor commands

Change the QEMU driver to not directly invoke the text mode monitor
APIs. Instead add a generic wrapper layer, which will eventually
invoke either the text or JSON protocol code as needed. Pass an
qemuMonitorPtr object into the monitor APIs instead of virDomainObjPtr
to complete the de-coupling of the monitor impl from virDomainObj
data structures

* src/qemu/qemu_conf.h: Remove qemuDomainObjPrivate definition
* src/qemu/qemu_driver.c: Add qemuDomainObjPrivate definition.
  Pass qemuMonitorPtr into all monitor APIs instead of the
  virDomainObjPtr instance.
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h: Add thin
  wrappers for all qemuMonitorXXX command APIs, calling into
  qemu_monitor_text.c/h
* src/qemu/qemu_monitor_text.c, src/qemu/qemu_monitor_text.h:
  Rename qemuMonitor -> qemuMonitorText & update to accept
  qemuMonitorPtr instead of virDomainObjPtr

Decouple the monitor code from the virDomainDefPtr structure
by moving the disk encryption lookup code back into the
qemu_driver.c file. Instead provide a function callback to
the monitor code which can be invoked to retrieve encryption
data as required.

* src/qemu/qemu_driver.c: Add findDomainDiskEncryption,
  and findVolumeQcowPassphrase. Pass address of the method
  findVolumeQcowPassphrase into qemuMonitorOpen()
* src/qemu/qemu_monitor.c: Associate a disk
  encryption function callback with the qemuMonitorPtr
  object.
* src/qemu/qemu_monitor_text.c: Remove findDomainDiskEncryption
  and findVolumeQcowPassphrase.

Introduce a new qemuDomainObjPrivate object which is used to store
the private QEMU specific data associated with each virDomainObjPtr
instance. This contains a single member, an instance of the new
qemuMonitorPtr object which encapsulates the QEMU monitor state.
The internals of the latter are private to the qemu_monitor* files,
not to be shown to qemu_driver.c

* src/qemu/qemu_conf.h: Definition of qemuDomainObjPrivate.
* src/qemu/qemu_driver.c: Register a functions for creating
  and freeing qemuDomainObjPrivate instances with the domain
  capabilities. Remove the qemudDispatchVMEvent() watch since
  I/O watches are now handled by the monitor code itself. Pass
  a new qemuHandleMonitorEOF() callback into qemuMonitorOpen
  to allow notification when the monitor quits.
* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h: Introduce
  the 'qemuMonitor' object. Temporarily add new APIs
  qemuMonitorWrite, qemuMonitorRead, qemuMonitorWaitForInput
  to allow text based monitor impl to perform I/O.
* src/qemu/qemu_monitor_text.c: Call APIs for reading/writing
  to monitor instead of accessing the file handle directly.

The qemu_driver.c code should not contain any code that interacts
with the QEMU monitor at a low level. A previous commit moved all
the command invocations out. This change moves out the code which
actually opens the monitor device.

* src/qemu/qemu_driver.c: Remove qemudOpenMonitor & methods called
  from it.
* src/Makefile.am: Add qemu_monitor.{c,h}
* src/qemu/qemu_monitor.h: Add qemuMonitorOpen()
* src/qemu/qemu_monitor.c: All code for opening the monitor