KVM: arm64: Finalize ID registers only once per VM

KVM: arm64: Finalize ID registers only once per VM

Owing to the ID registers being global to the VM, there is no point
in computing them more than once.  However, recent changes making
use of kvm_set_vm_id_reg() outlined that we repeatedly hammer
the ID registers when we shouldn't.

Gate the ID reg update on the VM having never run.

Fixes: 50e7cce81b9b2 ("KVM: arm64: Limit clearing of ID_{AA64PFR0,PFR1}_EL1.GIC to userspace irqchip")
Fixes: 5cb57a1aff755 ("KVM: arm64: Zero ID_AA64PFR0_EL1.GIC when no GICv3 is presented to the guest")
Closes: https://lore.kernel.org/r/aRHf6x5umkTYhYJ3@finisterre.sirena.org.uk
Reported-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Link: https://patch.msgid.link/20251110173010.1918424-1-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>