Change "None" to Property None

This changes to accept the string none as the value None,
as if an empty string was entered. This allows setting the
netvm to "None" as described in QubesOS/qubes-issues#3942

qubesadmin/base.py

@@ -275,6 +275,8 @@ class PropertyHolder(object):
                 value = value.name
             if value is None:
                 value = ''
+            if str(value).lower() == "none":
+                value = ''
             try:
                 self.qubesd_call(
                     self._method_dest,