feat(backup/VmBackup): regularly report speed in task info

@xen-orchestra/backups/_VmBackup.js

@@ -251,6 +251,20 @@ exports.VmBackup = class VmBackup {
 
     const timestamp = Date.now()
 
+    const progress = {
+      handle: setInterval(() => {
+        const { size } = sizeContainer
+        const timestamp = Date.now()
+        Task.info('transfer', {
+          speed: (size - progress.size) / (timestamp - progress.timestamp),
+        })
+        progress.size = size
+        progress.timestamp = timestamp
+      }, 5e3 * 60),
+      size: sizeContainer.size,
+      timestamp,
+    }
+
     await this._callWriters(
       writer =>
         writer.run({
@@ -261,6 +275,8 @@ exports.VmBackup = class VmBackup {
       'writer.run()'
     )
 
+    clearInterval(progress.handle)
+
     const { size } = sizeContainer
     const end = Date.now()
     const duration = end - timestamp