vhost-user: Add VHOST_USER_POSTCOPY_END message

vhost-user: Add VHOST_USER_POSTCOPY_END message

This message is sent just before the end of postcopy to get the
client to stop using userfault since we wont respond to any more
requests.  It should close userfaultfd so that any other pages
get mapped to the backing file automatically by the kernel, since
at this point we know we've received everything.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>