vdsm: fix RTC offset

Upon RTC update (hwclock --systohc) on the guest side, libvirt sends the event
with RTC offset from the qemu start -rtc value in seconds. The received offset
should not replace the previous value, but should be added to it. Since the
time update event is relative to the qemu start time, we should store the
initial value and use it for the calculation.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=956741
Change-Id: I27c70a53f64fb05607e93bffbac25fdee7d1cd2a
Signed-off-by: Peter V. Saveliev <peet@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14750
Tested-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>