DM: reboot: clear the data of 0xcf9 if cold reboot

Linux kernel CF9 reboot is doing things like:
  - read value from 0xcf9
  - mask out the value according to the reboot type (warm vs cold)
  - BIT OR the value with reboot code
  - write the value to 0xcf9

If there are two reboot event and the first one is cold
reboot and the second one is warm reboot, vsbl query
reboot type after the second one and will get code reboot
instead of warm reboot.

We should clear the data of 0xcf9 if guest request cold reboot.

Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>

Author: fyin1

devicemodel/hw/platform/pm.c

@@ -71,6 +71,11 @@ reset_handler(struct vmctx *ctx, int vcpu, int in, int port, int bytes,
 			error = vm_suspend(ctx, VM_SUSPEND_RESET);
 			assert(error == 0 || errno == EALREADY);
 		}
+
+		/* cold reset should clear the value in 0xcf9 */
+		if (reset_control & 0x8) {
+			reset_control = 0;
+		}
 	}
 	return 0;
 }