vhost/vdpa: use get_config_size callback in vhost_vdpa_config_validate()

commit fde5b51795fa87107c13092eb54f6632b3b2f70b from
https://github.com/altera-opensource/linux-socfpga.git



[upstream commit d6d8bb92]

Let's use the new 'get_config_size()' callback available instead of
using the 'virtio_id' to get the size of the device config space.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20210315163450.254396-10-sgarzare@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Wenlin Kang <wenlin.kang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>