backup: allow backup running VMs

Since we have LVM by default, it is possible to backup VMs while they
are running. For now it will include its state from before startup, but
later we may implement some other logic (a snapshot of running VM).

qubes/backup.py

@@ -705,8 +705,6 @@ class Backup(object):
             vm.features['backup-content'] = False
 
         for qid, vm_info in files_to_backup.items():
-            if qid != 0 and vm_info.vm.is_running():
-                raise qubes.exc.QubesVMNotHaltedError(vm_info.vm)
             # VM is included in the backup
             backup_app.domains[qid].features['backup-content'] = True
             backup_app.domains[qid].features['backup-path'] = vm_info.subdir