xen: set Sys.os_type to "Unix" instead of "xen"

The stdlib Filename module calls 'assert false' if it doesn't
recognise the os_type. Might as well be Unix.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #78 from djs55/rename-xen-to-Unix

xen: set Sys.os_type to "Unix" instead of "xen"