security_util: verify xattrs only if ref is present

After 7cfb7aab573 commit starting a domain pullutes logs with
warnings like [1]. The reason is resource files do not
have timestamp before starting a domain and after destroying
domain the timestamp is cleared. Let's check the timestamp
only if attribute with refcounter is found.

[1] warning : virSecurityValidateTimestamp:198 : Invalid XATTR timestamp detected on \
/some/path secdriver=dac

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>