FIXUP/HACK: Use internal snapshots for non-running VMs

This feels like a bug, but let's check with libvirt first:
cockpit-project#1280 (comment)

src/helpers.js

@@ -881,6 +881,12 @@ export function getStoragePoolPath(storagePools, poolName, connectionName) {
 }
 
 export function vmSupportsExternalSnapshots(config, vm, storagePools) {
+    // HACK/TODO: external snapshots fail for stopped VMs; intended? https://github.com/cockpit-project/cockpit-machines/pull/1280#issuecomment-1910102935
+    if (vm.state !== 'running') {
+        logDebug(`vmSupportsExternalSnapshots: vm ${vm.name} is not running, false`);
+        return false;
+    }
+
     const disks = Object.values(vm.disks);
 
     // If at leat one disk has internal snapshot preference specified, use internal snapshot for all disk,