src: remove use of g_date_time_new_from_iso8601 function

src: remove use of g_date_time_new_from_iso8601 function

The g_date_time_new_from_iso8601() function was introduced as
a replacement for strptime in

  commit 810613a60efe3924c536b3663246900bc08910a5
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Mon Dec 23 15:37:26 2019 +0000

    src: replace strptime()/timegm()/mktime() with GDateTime APIs set

Unfortunately g_date_time_new_from_iso8601 isn't available until
glib 2.56, and backporting it requires alot of code copying and
poking at private glib structs.

This reverts domain_conf.c back to its original parsing logic prior
to 810613a60efe3924c536b3663246900bc08910a5, but using g_date_time_new()
instead of gmtime(). The other files are then adapted to follow a
similar approach.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>