OvmfPkg/QemuFwCfgLib: generalize InternalQemuFwCfgDmaBytes() to SKIP op
    
    The fw_cfg DMA interface provides a simple method to skip over bytes in an
    fw_cfg blob before reading or writing more bytes.
    InternalQemuFwCfgDmaBytes() can support it easily, we just have to expose
    the Control parameter more flexibly than the current "Write" BOOLEAN.
    
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=359
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>