xen: Provide XEN_DMOP_remote_shutdown

xen: Provide XEN_DMOP_remote_shutdown

SCHEDOP_remote_shutdown should be a DMOP so that a deprivileged qemu
can do the propery tidying up.

We need to keep SCHEDOP_remote_shutdown for ABI stability reasons and
because it is needed for PV guests.

CC: Jan Beulich <jbeulich@suse.com>
CC: Andrew Cooper <andrew.cooper3@citrix.com>
CC: George Dunlap <George.Dunlap@eu.citrix.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Tim Deegan <tim@xen.org>
CC: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>