Revert "KVM: Support vCPU-based gfn->hva cache"

Revert "KVM: Support vCPU-based gfn->hva cache"

This reverts commit 4ecf9d9edae7e3ecbb6d191a08eef75ea7effb3c.

I've been sitting on this revert for too long and it unfortunately
missed 4.11.  It's also the reason why I haven't merged ring-based
dirty tracking for 4.12.

Using kvm_vcpu_memslots in kvm_gfn_to_hva_cache_init and
kvm_vcpu_write_guest_offset_cached means that the MSR value can
now be used to access SMRAM, simply by making it point to an SMRAM
physical address.  This is problematic because it lets the guest
OS overwrite memory that it shouldn't be able to touch.

Cc: stable@vger.kernel.org
Fixes: 4ecf9d9edae7e3ecbb6d191a08eef75ea7effb3c
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>