xc_interface_open etc. is not going to work if we have dropped
privilege, but xendevicemodel_shutdown will if everything is new
enough.

xendevicemodel_shutdown is only availabe in Xen 4.10 and later, so
provide a stub for earlier versions.
Signed-off-by: NIan Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: NAnthony PERARD <anthony.perard@citrix.com>