golang/xenlight: rename Ctx receivers to ctx

golang/xenlight: rename Ctx receivers to ctx

As a matter of style, it is strange to see capitalized receiver names,
due to the significance of capitalized symbols in Go (although there is
in fact nothing special about a capitalized receiver name). Fix this in
xenlight.go by running:

  gofmt -w -r 'Ctx -> ctx' xenlight.go

from tools/golang/xenlight. There is no functional change.

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