feat(vm.revert): can snapshot before (#395)

See vatesfr/xo-web#1445

src/api/vm.coffee

@@ -878,15 +878,12 @@ exports.resume = resume
 
 #---------------------------------------------------------------------
 
-# revert a snapshot to its parent VM
-revert = $coroutine ({snapshot}) ->
-  # Attempts a revert from this snapshot to its parent VM
-  yield @getXapi(snapshot).call 'VM.revert', snapshot._xapiRef
-
-  return true
+revert = ({snapshot, snapshotBefore}) ->
+  return @getXapi(snapshot).revertVm(snapshot._xapiId, snapshotBefore)
 
 revert.params = {
-  id: { type: 'string' }
+  id: { type: 'string' },
+  snapshotBefore: { type: 'boolean', optional: true }
 }
 
 revert.resolve = {

src/xapi/mixins/vm.js

@@ -326,5 +326,13 @@ export default {
 
   async editVm (id, props) {
     return /* await */ this._editVm(this.getObject(id), props)
+  },
+
+  async revertVm (snapshotId, snapshotBefore = true) {
+    const snapshot = this.getObject(snapshotId)
+    if (snapshotBefore) {
+      await this._snapshotVm(snapshot.$snapshot_of)
+    }
+    return this.call('VM.revert', snapshot.$ref)
   }
 }