OvmfPkg/AcpiPlatformDxe: implement the QEMU_LOADER_WRITE_POINTER command

The QEMU_LOADER_WRITE_POINTER command instructs the firmware to write the
address of a field within a previously allocated/downloaded fw_cfg blob
into another (writeable) fw_cfg file at a specific offset.

Put differently, QEMU_LOADER_WRITE_POINTER propagates, to QEMU, the
address that QEMU_LOADER_ALLOCATE placed the designated fw_cfg blob at, as
adjusted for the given field inside the allocated blob.

The implementation is similar to that of QEMU_LOADER_ADD_POINTER. Since
here we "patch" a pointer object in "fw_cfg file space", not guest memory
space, we utilize the QemuFwCfgSkipBytes() and QemuFwCfgWriteBytes() APIs
completed in commit range 465663e9f128..7fcb73541299.

An interesting aspect is that QEMU_LOADER_WRITE_POINTER creates a
host-level reference to a guest memory location. Therefore, if we fail to
process the linker/loader script for any reason, we have to clear out
those references first, before we release the guest memory allocations in
response to the error.

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: Jordan Justen <jordan.l.justen@intel.com>

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