i386/tdvf: Introduce function to parse TDVF metadata

i386/tdvf: Introduce function to parse TDVF metadata

TDX VM needs to boot with its specialized firmware, Trusted Domain
Virtual Firmware (TDVF). QEMU needs to parse TDVF and map it in TD
guest memory prior to running the TDX VM.

A TDVF Metadata in TDVF image describes the structure of firmware.
QEMU refers to it to setup memory for TDVF. Introduce function
tdvf_parse_metadata() to parse the metadata from TDVF image and store
the info of each TDVF section.

TDX metadata is located by a TDX metadata offset block, which is a
GUID-ed structure. The data portion of the GUID structure contains
only an 4-byte field that is the offset of TDX metadata to the end
of firmware file.

Select X86_FW_OVMF when TDX is enable to leverage existing functions
to parse and search OVMF's GUID-ed structures.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Co-developed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250508150002.689633-18-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>