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>

arch/x86/lguest/head_32.S

@@ -84,7 +84,7 @@ ENTRY(lg_irq_enable)
 	 * set lguest_data.irq_pending to X86_EFLAGS_IF.  If it's not zero, we
 	 * jump to send_interrupts, otherwise we're done.
 	 */
-	testl $0, lguest_data+LGUEST_DATA_irq_pending
+	cmpl $0, lguest_data+LGUEST_DATA_irq_pending
 	jnz send_interrupts
 	/*
 	 * One cool thing about x86 is that you can do many things without using