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.

vrtManager/network.py

@@ -442,12 +442,7 @@ class wvmNetwork(wvmConnect):
     def edit_network(self, new_xml):
         """Defines this network anew from new_xml: of the same name and, if it
         names one, the same UUID, not another network."""
-        # read for its name and UUID only, its entities not expanded: libvirt gets the text itself
-        parser = etree.XMLParser(resolve_entities=False, no_network=True)
-        try:
-            tree = etree.fromstring(new_xml.encode(), parser)  # bytes: the XML may declare its encoding
-        except etree.XMLSyntaxError as err:
-            raise util.OperationError(f"The XML is not valid: {err}") from None
+        tree = util.parse_xml(new_xml)
         if tree.tag != "network" or tree.findtext("name") != self.net.name():
             raise util.OperationError(f"The XML must define network {self.net.name()}")
         if tree.findtext("uuid") not in (None, self.net.UUIDString()):