qemu: stop checking virObjectUnref return value

qemu: stop checking virObjectUnref return value

Some, but not all, of the monitor event handlers check
the virObjectUnref return value to see if the domain
was disposed.

It should not be possible for this to happen, since
the function already holds a lock on the domain and
has only just acquired an extra reference on the
domain a few lines earlier.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>