qemu: validate: Validate maximum start time for <clock offset='absolute'>

qemu: validate: Validate maximum start time for <clock offset='absolute'>

Glib can internally convert only unix timestamps up to
9999-12-31T23:59:59 (253402300799). Validate that the user doesn't use
more than that as otherwise we cause an assertion failure:

 (process:1183396): GLib-CRITICAL **: 14:25:00.906: g_date_time_format: assertion 'datetime != NULL' failed

Additionally adjust the schema to allow bigger values as we use
'unsigned long long' to parse the value.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2128993
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>