vdpa_sim: make 'config' generic and usable for any device type

Add new 'config_size' attribute in 'vdpasim_dev_attr' and allocates
'config' dynamically to support any device types.

Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-12-sgarzare@redhat.com


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