Use the libvhost-user library.

This ended up being a rather large patch that cannot be easily splitted,
due to massive code move and API changes.

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>