arm64: KVM: Warn when PARange is less than 40 bits

We always thought that 40bits of PA range would be the minimum people
would actually build. Anything less is terrifyingly small.

Turns out that we were both right and wrong. Nobody has ever built
such a system, but the ARM Foundation Model has a PARange set to 36bits.
Just because we can. Oh well. Now, the KVM API explicitely says that
we offer a 40bit PA space to the VM, so we shouldn't run KVM on
the Foundation Model at all.

That being said, this patch offers a less agressive alternative, and
loudly warns about the configuration being unsupported. You'll still
be able to run VMs (at your own risks, though).

This is just a workaround until we have a proper userspace API where
we report the PARange to userspace.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>