i386/tdx: load TDVF for TD guest

i386/tdx: load TDVF for TD guest

TDVF(OVMF) needs to run at private memory for TD guest. TDX cannot
support pflash device since it doesn't support read-only private memory.
Thus load TDVF(OVMF) with -bios option for TDs.

Use memory_region_init_ram_guest_memfd() to allocate the MemoryRegion
for TDVF because it needs to be located at private memory.

Also store the MemoryRegion pointer of TDVF since the shared ramblock of
it can be discared after it gets copied to private ramblock.

Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
Co-developed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250508150002.689633-17-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>