KVM: x86/mmu: allow APICv memslot to be enabled but invisible

on AMD, APIC virtualization needs to dynamicaly inhibit the AVIC in a
response to some events, and this is problematic and not efficient to do by
enabling/disabling the memslot that covers APIC's mmio range.

Plus due to SRCU locking, it makes it more complex to
request AVIC inhibition.

Instead, the APIC memslot will be always enabled, but be invisible
to the guest, such as the MMU code will not install a SPTE for it,
when it is inhibited and instead jump straight to emulating the access.

When inhibiting the AVIC, this SPTE will be zapped.

This code is based on a suggestion from Sean Christopherson:
https://lkml.org/lkml/2021/7/19/2970



Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210810205251.424103-8-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>