qga: use fixed-length and GDateTime for log timestamp

qga: use fixed-length and GDateTime for log timestamp

The old code is kind of wrong. Say it's 1649309843.000001 seconds past
the epoch. Prints "1649309843.1". 9us later, it prints "1649309843.10".
Should really use %06lu for the microseconds part.

Use GDateTime instead, as suggested by Daniel.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>