conf: Introduce 'absolute' clock offset

conf: Introduce 'absolute' clock offset

The 'absolute' clock offset type has a 'start' attribute which is an
unix epoch timestamp to which the hardware clock is always set at start
of the VM.

This is useful if some VM needs to be kept set to an arbitrary time for
e.g. testing or working around broken software.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>