KVM: x86/mmu: Do not cache MMIO accesses while memslots are in flux

When installing new memslots, KVM sets bit 0 of the generation number to
indicate that an update is in-progress.  Until the update is complete,
there are no guarantees as to whether a vCPU will see the old or the new
memslots.  Explicity prevent caching MMIO accesses so as to avoid using
an access cached from the old memslots after the new memslots have been
installed.

Note that it is unclear whether or not disabling caching during the
update window is strictly necessary as there is no definitive
documentation as to what ordering guarantees KVM provides with respect
to updating memslots.  That being said, the MMIO spte code does not
allow reusing sptes created while an update is in-progress, and the
associated documentation explicitly states:

    We do not want to use an MMIO sptes created with an odd generation
    number, ...  If KVM is unlucky and creates an MMIO spte while the
    low bit is 1, the next access to the spte will always be a cache miss.

At the very least, disabling the per-vCPU MMIO cache during updates will
make its behavior consistent with the MMIO spte behavior and
documentation.

Fixes: 56f17dd3 ("kvm: x86: fix stale mmio cache bug")
Cc: <stable@vger.kernel.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>