vdpa_sim: add set_config callback in vdpasim_dev_attr

The set_config callback can be used by the device to parse the
config structure modified by the driver.

The callback will be invoked, if set, in vdpasim_set_config() after
copying bytes from caller buffer into vdpasim->config buffer.

Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-14-sgarzare@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>