USN-4572-2: Spice vulnerability

USN-4572-1 fixed a vulnerability in Spice. This update provides the corresponding update for Ubuntu 14.04 ESM. Original advisory details: Frediano Ziglio discovered that Spice incorrectly handled QUIC image decoding. A remote attacker could use this to cause Spice to crash, resulting in a denial of service, or possibly execute arbitrary code.