pc: Disable HPET for ISA machine

There was no HPET on ISA boxes.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/pc_piix.c

@@ -157,6 +157,7 @@ static void pc_init1(MemoryRegion *system_memory,
         pci_bus = NULL;
         i440fx_state = NULL;
         isa_bus_new(NULL);
+        no_hpet = 1;
     }
     isa_bus_irqs(isa_irq);