vircgroupv2: fix parsing multiple values in single file

vircgroupv2: fix parsing multiple values in single file

Our virStrToLong* helpers converts string to integers where it wraps
strtol standard function.  After the conversion happens and there are
some remaining invalid characters our helpers will fail if the second
argument is NULL.

We need to pass pointer to string in cases where there are multiple
values in a single file.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1741825

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