qemu-img info needs -U flag on running VMs

When getting VM disks informations on a running VM, the following error
occured:

    The minion function caused an exception: Traceback (most recent call last):
      File "/usr/lib/python3.6/site-packages/salt/minion.py", line 1673, in _thread_return
        return_data = minion_instance.executors[fname](opts, data, func, args, kwargs)
      File "/usr/lib/python3.6/site-packages/salt/executors/direct_call.py", line 12, in execute
        return func(*args, **kwargs)
      File "/usr/lib/python3.6/site-packages/salt/modules/virt.py", line 2411, in full_info
        'vm_info': vm_info()}
      File "/usr/lib/python3.6/site-packages/salt/modules/virt.py", line 2020, in vm_info
        info[domain.name()] = _info(domain)
      File "/usr/lib/python3.6/site-packages/salt/modules/virt.py", line 2004, in _info
        'disks': _get_disks(dom),
      File "/usr/lib/python3.6/site-packages/salt/modules/virt.py", line 465, in _get_disks
        output = _parse_qemu_img_info(qemu_output)
      File "/usr/lib/python3.6/site-packages/salt/modules/virt.py", line 287, in _parse_qemu_img_info
        raw_infos = salt.utils.json.loads(info)
      File "/usr/lib/python3.6/site-packages/salt/utils/json.py", line 92, in loads
        return json_module.loads(s, **kwargs)
      File "/usr/lib64/python3.6/json/__init__.py", line 354, in loads
        return _default_decoder.decode(s)
      File "/usr/lib64/python3.6/json/decoder.py", line 339, in decode
        obj, end = self.raw_decode(s, idx=_w(s, 0).end())
      File "/usr/lib64/python3.6/json/decoder.py", line 357, in raw_decode
        raise JSONDecodeError("Expecting value", s, err.value) from None
    json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

This is due to the fact that qemu-img can't get infos on a disk that is
already used like by a running VM. Using the qemu-img -U flag gets it
running in all cases.

salt/modules/virt.py

@@ -458,7 +458,7 @@ def _get_disks(dom):
             if driver is not None and driver.get('type') == 'qcow2':
                 try:
                     stdout = subprocess.Popen(
-                                ['qemu-img', 'info', '--output', 'json', '--backing-chain', disk['file']],
+                                ['qemu-img', 'info', '-U', '--output', 'json', '--backing-chain', disk['file']],
                                 shell=False,
                                 stdout=subprocess.PIPE).communicate()[0]
                     qemu_output = salt.utils.stringutils.to_str(stdout)

tests/unit/modules/test_virt.py

@@ -62,15 +62,16 @@ def setup_loader_modules(self):
         self.addCleanup(delattr, self, 'mock_libvirt')
         self.addCleanup(delattr, self, 'mock_conn')
         self.addCleanup(delattr, self, 'mock_popen')
-        mock_subprocess = MagicMock()
-        mock_subprocess.Popen.return_value = self.mock_popen  # pylint: disable=no-member
+        self.mock_subprocess = MagicMock()
+        self.mock_subprocess.return_value = self.mock_subprocess  # pylint: disable=no-member
+        self.mock_subprocess.Popen.return_value = self.mock_popen  # pylint: disable=no-member
         loader_globals = {
             '__salt__': {
                 'config.get': config.get,
                 'config.option': config.option,
             },
             'libvirt': self.mock_libvirt,
-            'subprocess': mock_subprocess
+            'subprocess': self.mock_subprocess
         }
         return {virt: loader_globals, config: loader_globals}
 
@@ -2661,6 +2662,11 @@ def test_full_info(self):
 
         actual = virt.full_info()
 
+        # Check that qemu-img was called with the proper parameters
+        qemu_img_call = [call for call in self.mock_subprocess.Popen.call_args_list if 'qemu-img' in call[0][0]][0]
+        self.assertIn('info', qemu_img_call[0][0])
+        self.assertIn('-U', qemu_img_call[0][0])
+
         # Test the hypervisor infos
         self.assertEqual(2816, actual['freemem'])
         self.assertEqual(6, actual['freecpu'])