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/instance.py

@@ -522,17 +522,6 @@ class wvmInstance(wvmConnect):
         title = util.get_xml_path(self._XMLDesc(PERSISTENT_XML if config else 0), "/domain/title")
         return title or ""
 
-    def get_filterrefs(self):
-        def filterrefs(ctx):
-            result = []
-            for net in ctx.xpath("/domain/devices/interface"):
-                filterref = net.xpath("filterref/@filter")
-                if filterref:
-                    result.append(filterref[0])
-            return result
-
-        return util.get_xml_path(self._XMLDesc(0), func=filterrefs)
-
     def get_description(self, config=False):
         description = util.get_xml_path(self._XMLDesc(PERSISTENT_XML if config else 0), "/domain/description")
         return description or ""