libvhost-user: introduce and use vu_has_protocol_feature()

This simplifies the various has_feature() checks, we already
have vu_has_feature() but it checks features, not protocol
features.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Message-Id: <20190904065021.1360-1-johannes@sipsolutions.net>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>