input: limit kbd queue depth

Apply a limit to the number of items we accept into the keyboard queue.

Impact: Without this limit vnc clients can exhaust host memory by
sending keyboard events faster than qemu feeds them to the guest.

Fixes: CVE-2017-8379
Cc: P J P <ppandit@redhat.com>
Cc: Huawei PSIRT <PSIRT@huawei.com>
Reported-by:  <jiangxin1@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170428084237.23960-1-kraxel@redhat.com
(cherry picked from commit fa18f36a)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>