xen: fix HYPERVISOR_dm_op() prototype

Change the third parameter to be the required struct xen_dm_op_buf *
instead of a generic void * (which blindly accepts any pointer).

Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>