Adds Ubuntu 16.04 as a supported guest os. This allows users to select
the OS when creating a template etc.
Note: As XenServer 6.5 does not have 16.04 in its list of known Ubuntu releases,
as a workaround 16.04 guest os refers to 14.04 for XenServer 6.5.
Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>