vhost-user: support registering external host notifiers

This patch introduces VHOST_USER_PROTOCOL_F_HOST_NOTIFIER.
With this feature negotiated, vhost-user backend can register
memory region based host notifiers. And it will allow the guest
driver in the VM to notify the hardware accelerator at the
vhost-user backend directly.

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>