Pino, you asked on IRC about an API to return a list of backends
through the API.
If this is for testing, you could add a debug-* API to do it.  These
are not part of the specified API and so we can change or remove them
at any time.
A canonical example of this is "debug-drives" which is used in various
guestfish tests, eg:
https://github.com/libguestfs/libguestfs/blob/master/fish/test-add-domain...
I have changed the output of this API several times and we could
remove it entirely in future.
Rich.
-- 
Richard Jones, Virtualization Group, Red Hat 
http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  
http://libguestfs.org