vDPA: only report driver features if FEATURES_OK is set

This commit reports driver features to user space
only after FEATURES_OK is features negotiation is done.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20220929014555.112323-3-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>