hw/i386/sev: Add function to get SEV metadata from OVMF header

hw/i386/sev: Add function to get SEV metadata from OVMF header

A recent version of OVMF expanded the reset vector GUID list to add
SEV-specific metadata GUID. The SEV metadata describes the reserved
memory regions such as the secrets and CPUID page used during the SEV-SNP
guest launch.

The pc_system_get_ovmf_sev_metadata_ptr() is used to retieve the SEV
metadata pointer from the OVMF GUID list.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Pankaj Gupta <pankaj.gupta@amd.com>
Message-ID: <20240530111643.1091816-19-pankaj.gupta@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>