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>