Don't kill QEMU process when a monitor I/O parsing error occurs

Don't kill QEMU process when a monitor I/O parsing error occurs

Currently whenever there is any failure with parsing the monitor,
this is treated in the same was as end-of-file (ie QEMU quit).
The domain is terminated, if not already dead.

With this change, failures in parsing the monitor stream do not
result in the death of QEMU. The guest continues running unchanged,
but all further use of the monitor will be disabled.

The VMM_FAILURE event will be emitted, and the mgmt application
can decide when to kill/restart the guest to re-gain control

* src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h: Run a
  different callback for monitor EOF vs error conditions.
* src/qemu/qemu_process.c: Emit VMM_FAILURE event when monitor
  fails