OvmfPkg/QemuFwCfgLib: add QemuFwCfgSkipBytes()

Introduce the new public API QemuFwCfgSkipBytes(), for advancing over
bytes in the selected firmware configuration item without transferring
data between the item and the caller.

When the DMA interface is available (the common case), the operation is
instantaneous. As a fallback, provide a loop of chunked reads into a small
stack-allocated scratch buffer.

This patch enables OvmfPkg/QemuFwCfgLib to overwrite part of a writeable
fw_cfg file, which will be particularly useful for the upcoming
QEMU_LOADER_WRITE_POINTER command in OvmfPkg/AcpiPlatformDxe.

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>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

OvmfPkg/Include/Library/QemuFwCfgLib.h

@@ -158,6 +158,22 @@ QemuFwCfgWriteBytes (
   );
 
 
+/**
+  Skip bytes in the firmware configuration item.
+
+  Increase the offset of the firmware configuration item without transferring
+  bytes between the item and a caller-provided buffer. Subsequent read, write
+  or skip operations will commence at the increased offset.
+
+  @param[in] Size  Number of bytes to skip.
+**/
+VOID
+EFIAPI
+QemuFwCfgSkipBytes (
+  IN UINTN                  Size
+  );
+
+
 /**
   Reads a UINT8 firmware configuration value