lguest: workaround cmpxchg8b_emu by ignoring cli in the guest.

It's only used by cmpxchg8b_emu (see db677ffa for the gory
details), and fixing that to be paravirt aware would be more work than
simply ignoring it (and AFAICT only help lguest).  This makes lguest
work on machines which have cmpxchg8b, for kernels compiled for older
processors.

(We can't emulate it properly: the popf which expects to restore interrupts
does not trap).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: virtualization@lists.osdl.org