vmspawn: Support direct kernel boot without UEFI firmware

vmspawn: Support direct kernel boot without UEFI firmware

When --linux= specifies a non-PE kernel image, automatically disable
UEFI firmware loading (as if --firmware= was passed). If --firmware=
is explicitly set to a path in this case, fail with an error. Booting
a UKI with --firmware= is also rejected since UKIs require UEFI.

--firmware= (empty string) can also be used explicitly to disable
firmware loading for PE kernels.

Other changes:
- Extract OVMF pflash drive setup into cmdline_add_ovmf()
- Extract kernel image type detection into determine_kernel()
- Add smbios_supported() helper to centralize the SMBIOS availability
  check (always available on x86, elsewhere requires firmware)
- Gate SMM, OVMF drives, SMBIOS11 and credential SMBIOS paths
  on firmware/SMBIOS being available
- Beef up the credential logic to fall back to fw_cfg and kernel
  command line in case SMBIOS is not available