feat(instances): a VM's TPM 2.0 is switched on and off on its Options tab

A VM made without a TPM, a Windows 10 to be upgraded to Windows 11, had
no way to get one but the XML editor. The Options tab has a TPM 2.0
switch, as the RNG one: it adds an emulated TPM of the host's model to
the definition for the next start, or removes it. Its state stays on the
host until the VM is undefined, so a TPM added again has it back. A TPM
the switch did not make (passthrough, 1.2, encrypted, with its own state
path) is left to the XML editor, and a host without swtpm says so.

instances/tests/test_live_xml_effects.py

@@ -130,6 +130,11 @@ EDITS = {
         lambda t: not t.xpath("devices/rng"),
         ["devices/rng"], True, False,
     ),
+    "set_tpm": (
+        lambda vm: vm.set_tpm(True),
+        lambda t: first(t, "devices/tpm/backend/@version") == "2.0",
+        ["devices/tpm"], True, False,
+    ),
     "set_on_crash": (
         lambda vm: vm.set_on_crash("restart"),
         lambda t: t.findtext("on_crash") == "restart",