KVM: arm64: Prevent NV feature flag on systems w/o nested virt

It would appear that userspace can select the NV feature flag regardless
of whether the system actually supports the feature. Obviously a nested
guest isn't getting far in this situation; let's reject the flag
instead.

Link: https://lore.kernel.org/r/20230920195036.1169791-6-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>