util: alloc: Introduce 'VIR_AUTOCLEAN' macros for clearing stack'd structs
    
    The new utility macros are useful for variables we put on the stack but
    require some cleanup. The most prominent of those is virBuffer which is
    used almost exclusively in that way.
    
    Signed-off-by: Peter Krempa <pkrempa@redhat.com>
    Reviewed-by: Eric Blake <eblake@redhat.com>