vhost-user: support protocol features

vhost-user: support protocol features

The two protocol features messages are introduced by qemu vhost
maintainer(Michael) for extendting vhost-user interface. Here is
an excerpta from the vhost-user spec:

    Any protocol extensions are gated by protocol feature bits,
    which allows full backwards compatibility on both master
    and slave.

The vhost-user multiple queue features will be treated as a vhost-user
extension, hence, we have to implement the two messages first.

VHOST_USER_PROTOCOL_FEATURES is initialized to 0, as we don't support
any yet.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Huawei Xie <huawei.xie@intel.com>