qemu_migration: Acquire correct job in qemuMigrationSrcIsAllowed

qemu_migration: Acquire correct job in qemuMigrationSrcIsAllowed

Commit 62627524607f added the acquiring of a job, but it is not always
VIR_ASYNC_JOB_MIGRATION_OUT, so the code fails when doing save or anything else.
Correct the async job by passing it from the caller as another parameter.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>