KVM: Ensure all vcpus are consistent with in-kernel irqchip settings

KVM: Ensure all vcpus are consistent with in-kernel irqchip settings

CVE-2012-1601

BugLink: http://bugs.launchpad.net/bugs/971685

If some vcpus are created before KVM_CREATE_IRQCHIP, then
irqchip_in_kernel() and vcpu->arch.apic will be inconsistent, leading
to potential NULL pointer dereferences.

Fix by:
- ensuring that no vcpus are installed when KVM_CREATE_IRQCHIP is called
- ensuring that a vcpu has an apic if it is installed after KVM_CREATE_IRQCHIP

This is somewhat long winded because vcpu->arch.apic is created without
kvm->lock held.

Based on earlier patch by Michael Ellerman.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Avi Kivity <avi@redhat.com>
(cherry picked from commit 3e515705a1f46beb1c942bb8043c16f8ac7b1e9e)

Signed-off-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>