vhost/vdpa: use get_config_size callback in vhost_vdpa_config_validate()

stable inclusion
from stable-v5.13
commit d6d8bb92
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5WXCZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d6d8bb92fdde6390037bf9da174ed3ab551c04d7


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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: Pengyuan Zhao <zhaopengyuan@hisilicon.com>