qemu: add external backend for tpm

Introduce a new backend type 'external' for connecting to a swtpm daemon
not managed by libvirtd.

Mostly in one commit, thanks to -Wswitch and the way we generate
capabilities.

https://bugzilla.redhat.com/show_bug.cgi?id=2063723

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

tests/domaincapsdata/qemu_6.0.0.aarch64.xml

@@ -171,6 +171,7 @@
       <enum name='backendModel'>
         <value>passthrough</value>
         <value>emulator</value>
+        <value>external</value>
       </enum>
       <enum name='backendVersion'>
         <value>1.2</value>