libxl: Add qxl vga interface support for upstream qemu

libxl: Add qxl vga interface support for upstream qemu

Usage:
vga="qxl"

Qxl vga support many resolutions that not supported by stdvga,
mainly the 16:9 ones and other high up to 2560x1600.
With QXL you can get improved performance and smooth video also
with high resolutions and high quality.
Require their drivers installed in the domU and spice used
otherwise act as a simple stdvga.

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>
Signed-off-by: Zhou Peng <zpengxen@gmail.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>