KVM: arm64: Tighten handling of unknown FGT groups

KVM: arm64: Tighten handling of unknown FGT groups

triage_sysreg_trap() assumes that it knows all the possible values
for FGT groups, which won't be the case as we start adding more
FGT registers (unless we add everything in one go, which is obviously
undesirable).

At the same time, it doesn't offer much in terms of debugging info
when things go wrong.

Turn the "__NR_FGT_GROUP_IDS__" case into a default, covering any
unhandled value, and give the kernel hacker a bit of a clue about
what's wrong (system register and full trap descriptor).

Signed-off-by: Marc Zyngier <maz@kernel.org>