qemu: drivePivot: Fix assumption when 'block-job-complete' fails

qemu: drivePivot: Fix assumption when 'block-job-complete' fails

QEMU does not abandon the mirror. The job carries on in the synchronised
phase and it might be either pivoted again or cancelled. The commit
hints that the described behavior was happening in a downstream version.

If the command returns false there are two possible options:
1) qemu did not reach the point where it would ask the block job to
pivot
2) pivotting failed in the actual qemu coroutine

If either of those would happen we return failure and reset the
condition that waits for the block job to complete. This makes the API
fail but in case where qemu would actually abandon the mirror the fact
is notified via the event and handled asynchronously.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1202704