lua: Various fixes to the bindings (thanks Jerome Vuarand).

See http://article.gmane.org/gmane.comp.lang.lua.general/95065

Note that this is incompatible with existing code.  You
have to do:

  local G = require "guestfs"
  local g = G.create ()

ie. give the module your own name ("G" in that example).

lua/examples/create_disk.lua

@@ -1,10 +1,10 @@
 -- Example showing how to create a disk image.
 
-require "guestfs"
+local G = require "guestfs"
 
-output = "disk.img"
+local output = "disk.img"
 
-g = Guestfs.create ()
+local g = G.create ()
 
 -- Create a raw-format sparse disk image, 512 MB in size.
 file = io.open (output, "w")