hw/arm/virt: Add always-on property to the virt board timer

The virt board has an arch timer, which is always on.  Emit the
"always-on" property to indicate to Linux that it can switch off the
periodic timer and reduces the amount of interrupts injected into a
guest.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-id: 1453204158-11412-1-git-send-email-christoffer.dall@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/arm/virt.c

diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 134f452..15658f4 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -293,6 +293,7 @@
         qemu_fdt_setprop_string(vbi->fdt, "/timer", "compatible",
                                 "arm,armv7-timer");
     }
+    qemu_fdt_setprop(vbi->fdt, "/timer", "always-on", NULL, 0);
     qemu_fdt_setprop_cells(vbi->fdt, "/timer", "interrupts",
                        GIC_FDT_IRQ_TYPE_PPI, ARCH_TIMER_S_EL1_IRQ, irqflags,
                        GIC_FDT_IRQ_TYPE_PPI, ARCH_TIMER_NS_EL1_IRQ, irqflags,