[HVM][LINUX][TOOLS] Make xm {shutdown|reboot} do something sensible for HVM

[HVM][LINUX][TOOLS] Make xm {shutdown|reboot} do something sensible for HVM
domains with PV drivers loaded.  This patch creates a new PV-on-HVM
module, reboot.ko, which, when loaded, creates control/reboot_module
in the store.  The tools notice this, and disable the watch which would
normally destroy HVM domains which are the target of an xm shutdown
command, allowing the reboot module to shut the domain down cleanly.

Signed-off-by: Tetsu Yamamoto <yamamoto.tetsu@jp.fujitsu.com>
(Checkin comments by Steven Smith <sos22@cam.ac.uk>)