net/virtio: support vhost-user protocol features

net/virtio: support vhost-user protocol features

This patch adds support for Vhost-user protocol features.
It is required to support protocol features that were not in
initial Vhost-user specification, such as reply-ack, MTU...

Also, this patch prevents Virtio multiqueue feature negotiation
if the slave does not support MQ protocol feature as stated
in Vhost-user specification:
"The multiple queues feature is supported only when the protocol
feature ``VHOST_USER_PROTOCOL_F_MQ`` (bit 0) is set."

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>