Generate a valid imagelabel even for type 'none'

Commit 2ce63c16 added imagelabel generation when relabeling is turned
off. But we weren't filling out the sensitivity for type 'none' labels,
resulting in an invalid label:

$ virsh managedsave domain
error: unable to set security context 'system_u:object_r:svirt_image_t'
on fd 28: Invalid argument

src/security/security_selinux.c

@@ -670,7 +670,14 @@ virSecuritySELinuxGenSecurityLabel(virSecurityManagerPtr mgr,
         break;
 
     case VIR_DOMAIN_SECLABEL_NONE:
-        /* no op */
+        if (virSecuritySELinuxMCSGetProcessRange(&sens,
+                                                 &catMin,
+                                                 &catMax) < 0)
+            goto cleanup;
+
+        if (VIR_STRDUP(mcs, sens) < 0)
+            goto cleanup;
+
         break;
 
     default: