grubby-bls: always escape the delimiter character used in sed commands
    
    The forward slash character is used as the delimiter for sed substitutions
    so this is escaped before calling sed.
    
    But this was only done before removing parameters from the kernel command
    and not when adding it. Use it for all the cases when the parameters are
    replaced in the update_args() function.
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>