vmspawn: add Intel TDX confidential VM support

vmspawn: add Intel TDX confidential VM support

Wire up --coco=tdx alongside the existing SEV-SNP path. TDX requires KVM
on x86_64, a raw TDVF firmware loaded via -bios (no pflash/NVRAM split),
kernel-irqchip=split, and the "host" CPU model since QEMU rejects named
models. Sets up the tdx-guest object and confidential-guest-support=tdx0.

TDX measurement is different from QEMU's kernel-hashes injection: TDX
provides runtime measurements via RTMRs, so the initial measurement only
covers the firmware, which then measures the rest of the boot chain into
those RTMRs (done by OVMF today). Therefore a restriction to direct
kernel boot isn't required either.

Signed-off-by: Paul Meyer <katexochen0@gmail.com>