usb/hid: add hid_pointer_activate, use it

usb/hid: add hid_pointer_activate, use it

HID reorganziation broke the usb tablet in windows xp.  The reason is
that xp activates idle before it starts polling, which creates a
chicken-and-egg issue:  We don't call hid_pointer_poll because there are
no pending events.  We don't get any events because the activation code
in hid_pointer_poll is never executed and thus all pointer events are
routed to the PS/2 mouse by qemu.

Fix this by creating a hid_pointer_activate function and call it from
usb-hid when the guest sets the idle state.

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