docs: document sysfs ABI for vDPA bus

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>