qemu: Clear async job when p2p migration fails early

qemu: Clear async job when p2p migration fails early

When p2p migration fails early because qemuMigrationIsAllowed or
qemuMigrationIsSafe say migration should be cancelled, we fail to clear
the migration-out async job. As a result of that, further APIs called
for the same domain may fail with Timed out during operation: cannot
acquire state change lock.

Reported by Guido Winkelmann.
(cherry picked from commit 837993d845a32bb222959a84d1c03a0c47f785be)