qemu: Avoid dangling migration-out job when client dies

qemu: Avoid dangling migration-out job when client dies

When a client which started non-p2p migration dies in a bad time, the
source libvirtd never clears the migration job and almost nothing can be
done with the domain without restarting the daemon. This patch makes use
of connection close callbacks and ensures that migration job is properly
discarded when the client disconnects.