usb-hid: add hid_has_events()

Add hid_has_events function, use it to figure whenever there are pending
events instead of checking and updating USBHIDState->changed.

Setting ->changed to 1 on init is removed, that should have absolutely
no effect as the initial state of ->idle is 0 so we report hid state
anyway until the guest configures some idle time.  Also should clear
->idle on reset.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
