vmspawn: allow opt-in Secure Boot firmware for coco

vmspawn: allow opt-in Secure Boot firmware for coco

CoCo firmware is stateless, so Secure Boot keys cannot be enrolled at
runtime: it only enforces Secure Boot with keys baked in at build time,
refusing unsigned images from the first boot. The default exclusion of
the enrolled-keys firmware feature hence keeps unsigned images bootable,
but it also makes firmware discovery fail on distros that only ship
SNP/TDX firmware with pre-enrolled keys (e.g. Fedora's TDVF). Drop the
rejection of --secure-boot=yes with --coco= and instead treat it as an
opt-in to such firmware, by lifting the enrolled-keys exclusion.

While at it, reject --efi-nvram-template= and an explicit
--efi-nvram-state= path with --coco=, which were silently ignored, as
stateless firmware has no NVRAM to instantiate or persist.

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