[hvm] Keep track of time offset between domain time and dom0 time.

[hvm] Keep track of time offset between domain time and dom0 time.
On each rtc time update from the hvm domain, we send an ioreq request
to qemu which then updates the time offset xenstore entry.  The time
offset is preserved across reboot and can be set on domain creation
by setting the rtc_timeoffset variable in a config file.

From: Andrei Petrov <andrei.petrov@xensource.com>
Signed-off-by: Christian Limpach <Christian.Limpach@xensource.com>