xen: use an hypercall for shutdown and reboot

When running as a Xen guest it's easier to use an hypercall in order
to do power management operations (power off, power cycle). Do this
for all supported guest types (HVM and PVH). Note that for HVM the
power operation could also be done using ACPI, but there's no reason
to differentiate between PVH and HVM.

While there fix the shutdown handler to properly differentiate between
power cycle and power off requests.

Reported by: Freddy DISSAUX
MFC: 1 week
Sponsored by: Citrix Systems R&D