virCgroupKillRecursive: Return -1 on failure condition

virCgroupKillRecursive sneakily initializes 'ret' to 0 rather than the
usual -1. 401030499bf moved an error condition but didn't actually
modify 'ret' return the proper error code.

Fixes: 401030499bf
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>

src/util/vircgroup.c

@@ -2591,6 +2591,7 @@ virCgroupKillRecursive(virCgroupPtr group, int signum)
     if (!backends || !backendAvailable) {
         virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                        _("no cgroup backend available"));
+        ret = -1;
         goto cleanup;
     }