xenserver: Add GPLv2 license text.

xenserver: Add GPLv2 license text.

xsconsole is being relicensed under GPLv2 so we need to include the text.

It would be more usual to name this file COPYING and to name the LGPLv2.1
that is already named LICENSE as COPYING.LIB, but some of the files pulled
in from XenServer say that their license is in a file named LICENSE.  I
don't expect that Citrix would be willing to change that, so it seems
better to keep LGPLv2.1 named LICENSE.

Signed-off-by: Ben Pfaff <blp@nicira.com>