remote: use g_auto for client RPC return parameters

remote: use g_auto for client RPC return parameters

Currently some, but not all, methods have a call to the
xdr_free function, for the 'ret' variable. This is done
on methods where there are complex structs containing
allocated memory. In other cases the structs contain
allocated memory, but the pointer is stolen, so xdr_free
is not called. In other cases no allocated memory is
present, so xdr_free.

This is hard to reason about, because the definition of
the struct is not visible in the client stubs.

Switch to use g_auto() for the 'ret' variable, which
means 'xdr_free' is always going to be called. Some
places now need to use g_steal_pointer as a result.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>