OvmfPkg: switch the AmdSev platform to the fw_cfg-only ACPI platform driver

OvmfPkg: switch the AmdSev platform to the fw_cfg-only ACPI platform driver

For consistency with the historical OvmfPkg* platforms, switch the
remotely attested, QEMU/KVM-only, AmdSev platform from the AcpiPlatformDxe
driver to the QemuFwCfgAcpiPlatformDxe driver.

No module remains dependent on XenPlatformLib, so remove the
XenPlatformLib class resolution too, from the DSC file.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210526201446.12554-5-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>