Allow to specify different time-of-day clock offsets for HVM guests.

Allow to specify different time-of-day clock offsets for HVM guests.

There are some usage scenarios in which differing user domains want
to be using different base TOD clocks. This patch adds the ability
to specify the base TOD time difference.  The patch also adds a
hook point to notify another entity when the domain changes this
offset. This might occur, for instance, on a Linux domain using
hwclock -w.

Signed-off-by: Ben Thomas <ben@virtualiron.com>