This patch introduces the host notifier support in
vhost-user-bridge. A new option (-H) is added to use
the host notifier. This is mainly used to test the
host notifier implementation in vhost user.

Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>