net/virtio: introduce backend data

net/virtio: introduce backend data

The goal of this patch is to introduce backend-specific
data in order to better isolate what is backend-specific
from what is generic to Virtio-user.

For now, only Vhost-user protocol features are moved to
Vhost-user backend data.

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