qemu: blockjob: Clean out disk mirror data after concluding the job

qemu: blockjob: Clean out disk mirror data after concluding the job

The 'disk->mirrorJob' and 'disk->mirrorState' fields need to be cleared
after a blockjob, but should be kept around while 'disk->mirror' is
still in place. As 'disk->mirror' is cleared only after conclusion of
the job in 'qemuBlockJobEventProcessConcluded()' we should be resetting
them only afterwards.

Move the code later, but since the job is unregistered from the disk we
need to store the pointer to the disk before concluding the job.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>