vdpa: add get_config_size callback in vdpa_config_ops

This new callback is used to get the size of the configuration space
of vDPA devices.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20210315163450.254396-9-sgarzare@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>