This function is unlike get_events in that it makes it easy to process
one event at a time.  This is useful in the mirroring test cases, where
we want to process just one event (BLOCK_JOB_ERROR) and leave the others
to a helper function.

Acked-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>