fix(nwfilters): a filter in use is not deleted, and a change replaces no other filter

Plan reviewed with Codex; libvirt's behaviour checked on a host.

- A filter is not deleted while a VM's interface references it, as the
  VM runs or as saved for its next boot (a stopped VM too), or while
  another filter references it: libvirt refuses only one a running VM
  uses, and a VM using a filter that references a deleted one does not
  start. The VMs are read from one listing, one or two XML reads each,
  instead of a connection per VM; the refusal names them.
- Create refuses a name or UUID the host has, clone a name in use or
  one outside the panel's names, and editing the XML must keep the
  filter's name and UUID: each would replace another filter or make a
  copy. Entered XML is read for these checks without expanding entities
  or taking a DTD (util.parse_xml, shared with the network editor),
  and a rule is one <rule> element.
- A rule or reference is deleted only while it reads as the page showed
  it, not by index and three attributes: a rule alike in those or a
  filter changed meanwhile loses nothing.
- A reference is added only to another filter of the host not
  referenced yet; the picker offers those.
- References keep their parameters, in order (a name may repeat); a
  rule shows every protocol element it has, a rule without one too.
- One change a POST (two would both start from the page's reading,
  the second undoing the first); every change redirects and reports its
  refusal as a message, a POST for a filter gone meanwhile going to the
  list; the catch-all that showed any error goes, and so does
  wvmInstance.get_filterrefs, which only the old delete check used. The list reads each filter's
  XML once from one listing; a filter's page reads its XML once and
  only the other filters' names; the per-filter XML dialog of the list
  goes (a filter's page shows its XML). The XML editors send their text
  untouched too.

networks/tests/test_network_changes.py

@@ -142,11 +142,11 @@ class NetworkChangeTests(SimpleTestCase):
         net.net.update.assert_not_called()
 
     def test_the_xml_editor_expands_no_entity(self):
-        # a file of the panel's host named by an external entity is not read into the check
+        # a file of the panel's host named by an external entity is not read: no DTD is taken
         net = self.network()
         xml = ('<!DOCTYPE network [<!ENTITY name SYSTEM "file:///etc/hostname">]>'
                "<network><name>&name;</name></network>")
-        with self.assertRaisesRegex(OperationError, "must define network lab"):
+        with self.assertRaisesRegex(OperationError, "may not declare a DTD"):
             net.edit_network(xml)
 
     def test_the_xml_editor_takes_an_encoding_declaration(self):