vircgroup: fix g_variant_new_parsed format string causing abort

The original code was incorrect and never tested because at the time of
implementing it the cgroup file `io.weight` was not available.

Resolves: https://issues.redhat.com/browse/RHEL-45185


Introduced-by: 9c1693ef
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>