i386/xen: implement HYPERVISOR_sched_op, SCHEDOP_shutdown

i386/xen: implement HYPERVISOR_sched_op, SCHEDOP_shutdown

It allows to shutdown itself via hypercall with any of the 3 reasons:
  1) self-reboot
  2) shutdown
  3) crash

Implementing SCHEDOP_shutdown sub op let us handle crashes gracefully rather
than leading to triple faults if it remains unimplemented.

In addition, the SHUTDOWN_soft_reset reason is used for kexec, to reset
Xen shared pages and other enlightenments and leave a clean slate for the
new kernel without the hypervisor helpfully writing information at
unexpected addresses.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
[dwmw2: Ditch sched_op_compat which was never available for HVM guests,
        Add SCHEDOP_soft_reset]
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>