vDPA/ifcvf: verify mandatory feature bits for vDPA

vDPA requres VIRTIO_F_ACCESS_PLATFORM as a must, this commit
examines this when set features.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210317094933.16417-7-lingshan.zhu@intel.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>