KVM: x86: don't disable APICv memslot when inhibited

Thanks to the former patches, it is now possible to keep the APICv
memslot always enabled, and it will be invisible to the guest
when it is inhibited

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: <20210810205251.424103-9-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>