golang/xenlight: do not negate ret when converting to Error

golang/xenlight: do not negate ret when converting to Error

Commit 871e51d2d4 changed the sign on the xenlight error types (making
the values negative, same as the C-generated constants), but failed to
remove the code changing the sign before casting to Error().  This
results in error strings like "libxl error: <x>", rather than the
correct message. Fix all occurrances of this by running:

  gofmt -w -r 'Error(-ret) -> Error(ret)' xenlight.go

from tools/golang/xenlight.

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