i8259: don't abort when trying to use level sensitive irqs

This is a guest-triggerable error, as seen when using Xenix 2.3.4.
Replace hw_error by LOG_UNIMPL, so that guests can continue.
With this patch, I can install and use Xenix 2.3.4a without any problem.
I can also start installation of Xenix 2.3.4q, but it fails due to not
finding an hard disk.

Signed-off-by: Hervé Poussineau <hpoussin@reactos.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/intc/i8259.c

@@ -265,7 +265,8 @@ static void pic_ioport_write(void *opaque, hwaddr addr64,
             s->init4 = val & 1;
             s->single_mode = val & 2;
             if (val & 0x08) {
-                hw_error("level sensitive irq not supported");
+                qemu_log_mask(LOG_UNIMP,
+                              "i8259: level sensitive irq not supported\n");
             }
         } else if (val & 0x08) {
             if (val & 0x04) {