This leads to cleaner code in usb-hid, and removes up to a 1000 calls / sec to
qemu_get_clock_ns(vm_clock) if idle-time is set to its default value of 0.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>