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/models.py

@@ -289,6 +289,10 @@ class Instance(models.Model):
     def config_rng(self) -> str | None:
         return self.proxy.get_rng()
 
+    @cached_property
+    def config_tpm(self) -> str | None:
+        return self.proxy.get_tpm()
+
     @cached_property
     def config_on_crash(self) -> str:
         return self.proxy.get_on_crash()