VT-d is a kind of extension to improve performace on virtualized systems. It's not mandatory for whole VT, but for TXT it is.
VT-x is a pure CPU thing, VT-d virtualizes I/O operations (DMA, Interrupt and IO device remapping), so it's more related to the IIO part of CPU.
The bug found in C1 is related to a semaphore which doesn't work properly and because there's no workaround for this erratum, it must be fixed with a new stepping.