vDPA/ifcvf: retire ifcvf_private_to_vf

This commit retires ifcvf_private_to_vf, because
the vf is already a member of the adapter,
so it could be easily addressed by adapter->vf.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20221125145724.1129962-12-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>