fix(backups/{Delta,Full}BackupWriter}): fix this._vmBackupDir access

May fix #6584

Introduced by 45dcb914b.

@xen-orchestra/backups/writers/DeltaBackupWriter.js

@@ -174,7 +174,7 @@ exports.DeltaBackupWriter = class DeltaBackupWriter extends MixinBackupWriter(Ab
     const { size } = await Task.run({ name: 'transfer' }, async () => {
       await Promise.all(
         map(deltaExport.vdis, async (vdi, id) => {
-          const path = `${this._backupDir}/${vhds[id]}`
+          const path = `${this._vmBackupDir}/${vhds[id]}`
 
           const isDelta = vdi.other_config['xo:base_delta'] !== undefined
           let parentPath

@xen-orchestra/backups/writers/FullBackupWriter.js

@@ -45,7 +45,7 @@ exports.FullBackupWriter = class FullBackupWriter extends MixinBackupWriter(Abst
     const basename = formatFilenameDate(timestamp)
 
     const dataBasename = basename + '.xva'
-    const dataFilename = this._backupDir + '/' + dataBasename
+    const dataFilename = this._vmBackupDir + '/' + dataBasename
 
     const metadata = {
       jobId: job.id,