KVM: SVM: Set/clear CR8 write interception when AVIC is (de)activated

In the Linux kernel, the following vulnerability has been resolved:

KVM: SVM: Set/clear CR8 write interception when AVIC is (de)activated

Explicitly set/clear CR8 write interception when AVIC is (de)activated to

fix a bug where KVM leaves the interception enabled after AVIC is

activated. E.g. if KVM emulates INIT=>WFS while AVIC is deactivated, CR8

will remain intercepted in perpetuity.

On its own, the dangling CR8 intercept is "just" a performance issue, but

combined with the TPR sync bug fixed by commit d02e48830e3f ("KVM: SVM:

Sync TPR from LAPIC into VMCB::V_TPR even if AVIC is active"), the danging

intercept is fatal to Windows guests as the TPR seen by hardware gets

wildly out of sync with reality.

Note, VMX isn't affected by the bug as TPR_THRESHOLD is explicitly ignored

when Virtual Interrupt Delivery is enabled, i.e. when APICv is active in

KVM's world. I.e. there's no need to trigger update_cr8_intercept(), this

is firmly an SVM implementation flaw/detail.

WARN if KVM gets a CR8 write #VMEXIT while AVIC is active, as KVM should

never enter the guest with AVIC enabled and CR8 writes intercepted.

[Squash fix to avic_deactivate_vmcb. - Paolo]