vdpa: Fix pointer math bug in vdpasim_get_config()

If "offset" is non-zero then we end up copying from beyond the end of
the config because of pointer math.  We can fix this by casting the
struct to a u8 pointer.

Fixes: 2c53d0f6

 ("vdpasim: vDPA device simulator")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20200406144552.GF68494@mwanda
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>