net: clean up network at qemu process termination

net: clean up network at qemu process termination

We don't clean up network if fails to parse "-device" parameters without
calling net_cleanup(). I touch a problem, the tap device which is
created by qemu-ifup script could not be removed by qemu-ifdown script.
Some similar problems also exist in vl.c

In this patch, if network initialization successes, a cleanup function
will be registered to be called at qemu process termination.

Signed-off-by: Amos Kong <akong@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>