usb keyboard: add event event queue

usb keyboard: add event event queue

This patch adds a event queue to the usb keyboard.  This makes sure the
guest will see all key events even if they come in bursts.  With this
patch applied sending Ctrl-Alt-Del using vncviewer's F8 menu works.
Also with autosuspend enabled the first keypress on a suspended keyboard
takes a little longer to be delivered to the guest because the usb bus
must be resumed first.  Without event queue this easily gets lost.

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