OvmfPkg/AmdSev: use GenericQemuLoadImageLib in AmdSev builds

OvmfPkg/AmdSev: use GenericQemuLoadImageLib in AmdSev builds

Newer kernels support efistub and therefore don't need all the legacy
stuff in X86QemuLoadImageLib, which are harder to secure.  Specifically
the verification of kernel/initrd/cmdline blobs will be added only to
the GenericQemuLoadImageLib implementation, so use that for SEV builds.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ashish Kalra <ashish.kalra@amd.com>
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: Min Xu <min.m.xu@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3457
Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>