configs/x86: Add check for VT-d existence

Add a check that validates the existence of configured VT-d units.
Require that every config contains at least one unit. Experimental VT-d
emulation, including interrupt remapping support, is now available for
QEMU. So we can set the config fields accordingly. If VT-d is disabled
or a QEMU version without that support is used, the hypervisor continues
to ignore this error.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Update TODO

Remove recently added features, add VT-d emulation and a check for
virtualized environment (safety requirement).

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>