lguest: fix pending interrupt test.

Denys says:
  TEST with zero will always set ZF. Thus, "jnz send_interrupts" never jumps.

We get interrupts regularly enough that this didn't cause immediate problems.

Reported-by: Denys Vlasenko <vda.linux@googlemail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>