VirtualMachine: to_xml(): Add pretty_print keyword

If the `lxml` package is available, its ElementTree implementation is
used instead of the one in the Python standard library. This
implementation provides a "pretty print" option when serializing XML
documents.

Note that since `Element.attrib` is not a "real" dictionary object, it
does not accept keyword arguments to its `update` method, so a
dictionary must be passed in instead. This is used in a few places to
set element attributes in bulk.

src/mkvm/vmdesc.py

@@ -1,10 +1,16 @@
 from . import storage
-from xml.etree import ElementTree as etree
 import logging
 import os
 import re
 import tempfile
 import yaml
+try:
+    import lxml
+except ImportError:
+    lxml = None
+    from xml.etree import ElementTree as etree
+else:
+    from lxml import etree
 
 
 log = logging.getLogger(__name__)
@@ -272,7 +278,7 @@ class VirtualMachine(object):
             storage.deactivate_vg(self.vg_name)
         self._disk.deactivate()
 
-    def to_xml(self):
+    def to_xml(self, pretty_print=False):
         root = etree.Element('domain', type='kvm')
         etree.SubElement(root, 'name').text = self.name
 
@@ -357,22 +363,25 @@ class VirtualMachine(object):
         if self.video == 'qxl':
             elm_video = etree.SubElement(elm_devices, 'video')
             elm_model = etree.SubElement(elm_video, 'model')
-            elm_model.attrib.update(
+            elm_model.attrib.update(dict(
                 type=self.video,
                 ram='65536',
                 vram='65536',
                 heads='1',
-            )
+            ))
         elif self.video:
             elm_video = etree.SubElement(elm_devices, 'video')
             elm_model = etree.SubElement(elm_video, 'model')
-            elm_model.attrib.update(
+            elm_model.attrib.update(dict(
                 type=self.video,
                 vram='9216',
                 heads='1',
-            )
+            ))
 
-        return etree.tostring(root, encoding='unicode')
+        kwargs = {'encoding': 'unicode'}
+        if lxml:
+            kwargs['pretty_print'] = pretty_print
+        return etree.tostring(root, **kwargs)
 
     def _partition_gpt(self):
         storage.partition_gpt(self._disk.blockdev, self.partitions)