vDPA/ifcvf: get_driver_features from virtio registers

This commit implements a new function ifcvf_get_driver_feature()
which read driver_features from virtio registers.

To be less ambiguous, ifcvf_set_features() is renamed to
ifcvf_set_driver_features(), and ifcvf_get_features()
is renamed to ifcvf_get_dev_features() which returns
the provisioned vDPA device features.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230526145254.39537-3-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>