There's room for speeding up emulation of invalid guest state.  Let's
track our progress by adding a few small performance tests.

Current results on a Xeon E5 (Sandy Bridge) with unrestricted_guest=0:

    739 cycles/emulated jump instruction
    925 cycles/emulated move instruction
    887 cycles/emulated arithmetic instruction
    916 cycles/emulated memory load instruction
    1109 cycles/emulated memory RMW instruction

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