docs: document sysfs ABI for vDPA bus

mainline inclusion
from mainline-v5.17-rc1
commit 9c25cdeb
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5WXCZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9c25cdeb5f3ca9ef0d683ee8c29b7cb61a174165



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Add missing documentation of sysfs ABI for vDPA bus in
the new Documentation/ABI/testing/sysfs-bus-vdpa file.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20211126164753.181829-2-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>