ts-xen-install: enable timestamp on guests logs

ts-xen-install: enable timestamp on guests logs

Enable the timestamp feature of xenconsoled so guests logs have a time
reference. Can be helpful when debugging boot related slowness.

This requires using the XENCONSOLED_ARGS option and setting both the
log and the timestamp options. Note that setting XENCONSOLED_TRACE
will override any options in XENCONSOLED_ARGS, so they can not be used
in conjunction.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <iwj@xenproject.org>