qemu: fix race between disk mirror fail and cancel

qemu: fix race between disk mirror fail and cancel

If a VM migration is aborted, a disk mirror may be failed by QEMU before
libvirt has a chance to cancel it. The disk->mirrorState remains at
_ABORT in this case, and this breaks subsequent mirrorings of that disk.

We should instead check the mirrorState directly and transition to _NONE
if it is already aborted. Do the check *after* aborting the block job in
QEMU to avoid a race.

Signed-off-by: Michael Chapman <mike@very.puzzling.org>