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>
