vhost-user: add vhost_user_requires_shm_log()

Check if the backend has VHOST_USER_PROTOCOL_F_LOG_SHMFD feature and
require a shared log.

Signed-off-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>
Tested-by: Thibaut Collet <thibaut.collet@6wind.com>
