KVM: PIT: support mode 4

The in-kernel PIT emulation ignores pending timers if operating under
mode 4, which for example DragonFlyBSD uses (and Plan9 too, apparently).

Mode 4 seems to be similar to one-shot mode, other than the fact that it
starts counting after the next CLK pulse once programmed, while mode 1
starts counting immediately, so add a FIXME to enhance precision.

Fixes sourceforge bug 1952988.
Signed-off-by: NMarcelo Tosatti <mtosatti@redhat.com>
Acked-by: NSheng Yang <sheng.yang@intel.com>
Signed-off-by: NAvi Kivity <avi@qumranet.com>

arch/x86/kvm/i8254.c

@@ -288,6 +288,8 @@ static void pit_load_count(struct kvm *kvm, int channel, u32 val)
 	 * mode 1 is one shot, mode 2 is period, otherwise del timer */
 	switch (ps->channels[0].mode) {
 	case 1:
+        /* FIXME: enhance mode 4 precision */
+	case 4:
 		create_pit_timer(&ps->pit_timer, val, 0);
 		break;
 	case 2: