KVM: arm64: Make ICC_*SGI*_EL1 undef in the absence of a vGICv3
In the Linux kernel, the following vulnerability has been resolved:

KVM: arm64: Make ICC_*SGI*_EL1 undef in the absence of a vGICv3

On a system with a GICv3, if a guest hasn't been configured with
GICv3 and that the host is not capable of GICv2 emulation,
a write to any of the ICC_*SGI*_EL1 registers is trapped to EL2.

We therefore try to emulate the SGI access, only to hit a NULL
pointer as no private interrupt is allocated (no GIC, remember?).

The obvious fix is to give the guest what it deserves, in the
shape of a UNDEF exception.