Exclude stderr from returned output when executing powerpc-utils tools
    
    The GRUB2 class updateNVRAMBootList() method calls to some tools to update
    ppc64le's boot device order list but it doesn't exclude the error messages
    which leads to an error message being stored in the NVRAM instead of the
    boot list.
    
    To prevent this to happen, exclude the standard error messages from the
    output when calling the tools.
    
    Related: rhbz#1847604
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>