Keep the value of filesystem/source when attribute is changed

When the attribute of filesystem/source is changed the old value was
not assigned to the new attribute.

tests/xmlparse-xml/change-filesystems-out.xml

@@ -47,8 +47,8 @@
       <source usage="123" units="KiB"/>
       <target dir="/bar/baz"/>
     </filesystem>
-    <filesystem type="block">
-      <source dev="/dev/new"/>
+    <filesystem type="file">
+      <source dev="/dev/new" file="/dev/new"/>
       <target dir="/bar/baz"/>
       <readonly/>
     </filesystem>

tests/xmlparse.py

@@ -781,9 +781,9 @@ class XMLParseTest(unittest.TestCase):
         check("units", "MB", "KiB")
 
         check = self._make_checker(dev6)
-        check("type", "block")
         check("source", "/foo/bar", "/dev/new")
         check("readonly", False, True)
+        check("type", "block", "file")
 
         check = self._make_checker(dev7)
         check("type", "file")

virtinst/devicefilesystem.py

@@ -54,7 +54,7 @@ class VirtualFilesystem(VirtualDevice):
         DRIVER_DEFAULT]
 
 
-    type = XMLProperty("./@type",
+    _type_prop = XMLProperty("./@type",
                        default_cb=lambda s: None,
                        default_name=TYPE_DEFAULT)
     accessmode = XMLProperty("./@accessmode",
@@ -111,6 +111,25 @@ class VirtualFilesystem(VirtualDevice):
         return setattr(self, self._type_to_source_prop(), val)
     source = property(_get_source, _set_source)
 
+    def _get_type(self):
+        return getattr(self, '_type_prop')
+    def _set_type(self, val):
+        # Get type/value of the attrubute of "source" property
+        old_source_type = self._type_to_source_prop()
+        old_source_value = self.source
+
+        # Update "type" property
+        new_type = setattr(self, '_type_prop', val)
+
+        # If the attribute type of 'source' property has changed
+        # restore the value
+        if old_source_type != self._type_to_source_prop():
+            self.source = old_source_value
+
+        return new_type
+
+    type = property(_get_type, _set_type)
+
     def set_defaults(self, guest):
         ignore = guest