golang/xenlight: add error return type to Context.Cpupoolinfo

golang/xenlight: add error return type to Context.Cpupoolinfo

A previous commit that removed Context.CheckOpen revealed
an ineffectual assignent to err in Context.Cpupoolinfo, as
there is no error return type.

Since it appears that the intent is to return an error here,
add an error return value to the function signature.

Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>