KVM: arm64: nv: Add sanitising to VNCR-backed FGT sysregs

Fine Grained Traps are controlled by a whole bunch of features.
Each one of them must be checked and the corresponding masks
computed so that we don't let the guest apply traps it shouldn't
be using.

This takes care of HFG[IRW]TR_EL2, HDFG[RW]TR_EL2, and HAFGRTR_EL2.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20240214131827.2856277-6-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>