vdpa: add get_config_size callback in vdpa_config_ops

stable inclusion
from stable-v5.13
commit 442706f9
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=442706f9f94d28fe3c9f188ae4ebbd6b40addffe



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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>
Signed-off-by: Pengyuan Zhao <zhaopengyuan@hisilicon.com>