Add admin.vm.volume.Clear call (QubesOS/qubes-issues#5946)

qubesadmin/storage.py

@@ -236,6 +236,10 @@ class Volume(object):
             'ImportWithSize', payload=size_line.encode(),
             payload_stream=stream)
 
+    def clear_data(self):
+        ''' Clear existing volume content. '''
+        self._qubesd_call('Clear')
+
     def clone(self, source):
         ''' Clone data from sane volume of another VM.
 

qubesadmin/tools/qvm_template_postprocess.py

@@ -124,8 +124,7 @@ def import_root_img(vm, source_dir):
 
 def reset_private_img(vm):
     '''Clear private volume'''
-    with open('/dev/null', 'rb') as null:
-        vm.volumes['private'].import_data(stream=null)
+    vm.volumes['private'].clear_data()
 
 
 def import_appmenus(vm, source_dir):