QEMU monitor commands may sleep for a prolonged period of time.
If the virDomainObjPtr or qemu driver lock is held this will
needlessly block execution of many other API calls. it also
prevents asynchronous monitor events from being dispatched
while a monitor command is executing, because deadlock will
ensure.

To resolve this, it is neccessary to release all locks while
executing a monitor command. This change introduces a flag
indicating that a monitor job is active, and a condition
variable to synchronize access to this flag. This ensures that
only a single thread can be making a state change or executing
a monitor command at a time, while still allowing other API
calls to be completed without blocking

* src/qemu/qemu_driver.c: Release driver and domain lock when
  running monitor commands. Re-add locking to disk passphrase
  callback
* src/qemu/THREADS.txt: Document threading rules