Bug#807774: mesa: please enable virgl mesa driver

[Pedretti Fabio]

qemu 2.5 is in testing now, while linux 4.4 is in experimental.

More info about virgl here:
http://kernelnewbies.org/Linux_4.4#head-2eeb49fa6ff17e43ad0a5bf743c945a9289a58f4