VMCS shadow high access consistency

Test for consistency of mixed accesses to full and high parts of
64-bit VMCS fields. (Currently just tests TSC_OFFSET, which is one of
the fields shadowed by kvm.)

Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>