src: replace gmtime_r/localtime_r/strftime with GDateTime

src: replace gmtime_r/localtime_r/strftime with GDateTime

gmtime_r/localtime_r are mostly used in combination with
strftime to format timestamps in libvirt. This can all
be replaced with GDateTime resulting in simpler code
that is also more portable.

There is some boundary condition problem in parsing POSIX
timezone offsets in GLib which tickles our test suite.
The test suite is hacked to avoid the problem. The upsteam
GLib bug report is

  https://gitlab.gnome.org/GNOME/glib/issues/1999

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