KVM: arm64: vgic: Hoist SGI/PPI alloc from vgic_init() to kvm_create_vgic()

KVM: arm64: vgic: Hoist SGI/PPI alloc from vgic_init() to kvm_create_vgic()

If userspace creates vcpus, then a vgic, we end-up in a situation
where irqchip_in_kernel() will return true, but no private interrupt
has been allocated for these vcpus. This situation will continue
until userspace initialises the vgic, at which point we fix the
early vcpus. Should a vcpu run or be initialised in the interval,
bad things may happen.

An obvious solution is to move this fix-up phase to the point where
the vgic is created. This ensures that from that point onwards,
all vcpus have their private interrupts, as new vcpus will directly
allocate them.

With that, we have the invariant that when irqchip_in_kernel() is
true, all vcpus have their private interrupts.

Reported-by: Alexander Potapenko <glider@google.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250212182558.2865232-3-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>