OvmfPkg: create a SEV secret area in the AmdSev memfd

OvmfPkg: create a SEV secret area in the AmdSev memfd

SEV needs an area to place an injected secret where OVMF can find it
and pass it up as a ConfigurationTable.  This patch implements the
area itself as an addition to the SEV enhanced reset vector table using
an additional guid (4c2eb361-7d9b-4cc3-8081-127c90d3d294).

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3077
Signed-off-by: James Bottomley <jejb@linux.ibm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20201130202819.3910-5-jejb@linux.ibm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
[lersek@redhat.com: fix typo in "ResetVectorVtf0.asm" comments]