KVM: arm64: vgic: Make vgic_get_irq() more robust

KVM: arm64: vgic: Make vgic_get_irq() more robust

vgic_get_irq() has an awkward signature, as it takes both a kvm
*and* a vcpu, where the vcpu is allowed to be NULL if the INTID
being looked up is a global interrupt (SPI or LPI).

This leads to potentially problematic situations where the INTID
passed is a private interrupt, but that there is no vcpu.

In order to make things less ambiguous, let have *two* helpers
instead:

- vgic_get_irq(struct kvm *kvm, u32 intid), which is only concerned
  with *global* interrupts, as indicated by the lack of vcpu.

- vgic_get_vcpu_irq(struct kvm_vcpu *vcpu, u32 intid), which can
  return *any* interrupt class, but must have of course a non-NULL
  vcpu.

Most of the code nicely falls under one or the other situations,
except for a couple of cases (close to the UABI or in the debug code)
where we have to distinguish between the two cases.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20241117165757.247686-3-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>