vDPA/ifcvf: introduce get_dev_type() which returns virtio dev id

This commit introduces a new function get_dev_type() which returns
the virtio device id of a device, to avoid duplicated code.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210812032454.24486-2-lingshan.zhu@intel.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>