docs: add a page describing support guarantees for libvirt features

docs: add a page describing support guarantees for libvirt features

While we have collective knowledge about the support status of various
parts of libvirt, this has never been formally documented, leaving our
users to guess.

Note, this document makes one change to our previous policy. It explicitly
declares the RPC protocol of libvirtd as being a supported interface. THis
accepts the reality that we can a) never change it without breaking compat
with old libvirt.so, b) there are both rust + go impls that are written
against the RPC protocol already.

Reviewed-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>