KVM: x86: Add memory barrier on vmcs field lookup

This adds a memory barrier when performing a lookup into
the vmcs_field_to_offset_table.  This is related to
CVE-2017-5753.

Signed-off-by: Andrew Honig <ahonig@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>