libvhost-user: support host notifier

This patch introduces the host notifier support in
libvhost-user. A new API is added to support setting
host notifier for each queue.

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>