Queue some mouse events.

Queue some mouse events.

Inspired by xen-unstable changeset, but entirely rewritten:

  changeset:   17003:96b418cf047e6d869740efd3660ab175303a3148
  user:        Keir Fraser <keir.fraser@citrix.com>
  date:        Mon Feb 11 09:57:38 2008 +0000
  files:       tools/ioemu/hw/usb-hid.c tools/ioemu/sdl.c
  description:
  qemu: Queue mouse clicks.

  qemu doesn't enqueue mouse events, just records the latest mouse
  state. This can cause some lost mouse double clicks if the events are
  not processed fast enought. This patch implements a simple queue for
  left mouse click events.

  Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>