qemu: minor monitor lock cleanups

qemu: minor monitor lock cleanups

If virCondInit fails (okay, so that's unlikely), then we end up
attempting a virObjectUnlock() on the cleanup path, even though
we don't hold a lock.  This is not guaranteed to be safe.  While
at it, I noticed a couple places where we were referencing mon->fd
outside locks.

* src/qemu/qemu_monitor.c (qemuMonitorOpenInternal): Minimize lock
duration.  mon->watch doesn't need clean up on error.
(qemuMonitorGetBlockExtent, qemuMonitorBlockResize): Don't
dereference fd outside of lock.