OvmfPkg: AcpiPlatformDxe: don't rely on unstable QEMU interface

OvmfPkg: AcpiPlatformDxe: don't rely on unstable QEMU interface

The fw_cfg file "etc/acpi/tables" is not a stable guest interface -- QEMU
could rename it in the future, and/or introduce additional fw_cfg files
with ACPI payload. Only the higher-level "etc/table-loader" file is
considered stable, which contains a sequence of commands to assist
firmware with reading QEMU ACPI tables from the FwCfg interface.

Because edk2 provides publishing support for ACPI tables, OVMF only uses
the Allocate command to find the names of FwCfg files to read and publish
as ACPI tables.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15574 6f19259b-4bc3-4df7-8a09-765794883524