OvmfPkg: introduce QemuFwCfgSimpleParserLib
    
    We already parse some boolean and integer values from named fw_cfg files
    (usually into PCDs), and we're going to cover more. Add a dedicated
    library for centralizing the parsing logic.
    
    Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Per Sundstrom <per_sundstrom@yahoo.com>
    Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2681
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>