storage: gluster: Report error if no volumes were found in pool lookup

Similarly to the 'netfs' pool, return an error if the host does not have
any volumes.

src/storage/storage_backend_gluster.c

@@ -490,6 +490,7 @@ virStorageBackendGlusterFindPoolSources(virConnectPtr conn ATTRIBUTE_UNUSED,
                                     };
     virStoragePoolSourcePtr source = NULL;
     char *ret = NULL;
+    int rc;
     size_t i;
 
     virCheckFlags(0, NULL);
@@ -510,11 +511,18 @@ virStorageBackendGlusterFindPoolSources(virConnectPtr conn ATTRIBUTE_UNUSED,
         goto cleanup;
     }
 
-    if (virStorageBackendFindGlusterPoolSources(source->hosts[0].name,
-                                                0, /* currently ignored */
-                                                &list) < 0)
+    if ((rc = virStorageBackendFindGlusterPoolSources(source->hosts[0].name,
+                                                      0, /* currently ignored */
+                                                      &list)) < 0)
         goto cleanup;
 
+    if (rc == 0) {
+        virReportError(VIR_ERR_OPERATION_FAILED,
+                       _("no storage pools were found on host '%s'"),
+                       source->hosts[0].name);
+        goto cleanup;
+    }
+
     if (!(ret = virStoragePoolSourceListFormat(&list)))
         goto cleanup;