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