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/tests/test_xpath_values.py

@@ -10,16 +10,11 @@ if not settings.configured:
 
 from vrtManager.connection import wvmConnect
 from vrtManager.network import wvmNetwork
-from vrtManager.nwfilters import wvmNWFilter
 
 CAPS = """<capabilities><guest><arch name='x86_64'><wordsize>64</wordsize><emulator>/usr/bin/qemu-x86</emulator>
 <machine canonical='pc-q35'>q35</machine></arch>
 <arch name='aarch64'><emulator>/usr/bin/qemu-arm</emulator><machine>virt</machine></arch></guest></capabilities>"""
 
-FILTER = """<filter name='f'><rule action='drop' direction='in' priority='1'><tcp/></rule>
-<rule action='drop' direction='in' priority='2'><udp/></rule></filter>"""
-
-
 class FakeConn:
     def __init__(self):
         self.defined = None
@@ -41,20 +36,6 @@ class XPathValueTests(unittest.TestCase):
         self.assertEqual(self.conn.get_machine_types(injected), [])
         self.assertEqual(self.conn.get_capabilities(injected), {})
 
-    def test_nwfilter_rule_is_matched_by_its_attributes(self):
-        nwf = wvmNWFilter.__new__(wvmNWFilter)
-        nwf._XMLDesc = lambda flags: FILTER
-        # values with quotes or brackets are compared, not parsed as XPath
-        for action in ("drop'][@priority='2", "it's"):
-            with self.subTest(action=action), self.assertRaises(ValueError):
-                nwf.delete_rule(0, action, "in", "1")
-        kept = nwf.delete_rule(1, "drop", "in", "2")
-        self.assertEqual(kept.count("<rule "), 1)
-        self.assertIn("priority=\"1\"", kept)
-        # add_rule adds a rule of its own
-        added = nwf.add_rule("<rule action='drop' direction='in' priority='2'><icmp/></rule>")
-        self.assertEqual(added.count("<rule "), 3)
-
     def test_qos_direction_must_be_known(self):
         net = wvmNetwork.__new__(wvmNetwork)
         net.wvm = FakeConn()