hw/pl031: Actually raise interrupt on timer expiry

Fix a typo in pl031_interrupt() which meant we were setting a bit
in the interrupt mask rather than the interrupt status register
and thus not actually raising an interrupt. This fix allows the
rtctest program from the kernel's Documentation/rtc.txt to pass
rather than hanging.

Reported-by: Daniel Forsgren <daniel.forsgren@enea.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Andreas Färber <afaerber@suse.de>

hw/pl031.c

@@ -76,7 +76,7 @@ static void pl031_interrupt(void * opaque)
 {
     pl031_state *s = (pl031_state *)opaque;
 
-    s->im = 1;
+    s->is = 1;
     DPRINTF("Alarm raised\n");
     pl031_update(s);
 }