Fix "vargant package" for LXC 2.1+ config format

New config format would be under `lxc.rootfs.path` and be prefixed by
`dir:`.

lib/vagrant-lxc/driver.rb

@@ -50,20 +50,20 @@ module Vagrant
       end 
 
       def rootfs_path
-        config_entry = config_string.match(/^lxc\.rootfs\s+=\s+(.+)$/)[1]
-        case config_entry
-        when /^overlayfs:/
+        pathtype, path = config_string.match(/^lxc\.rootfs(?:\.path)?\s+=\s+(.+:)?(.+)$/)[1..2]
+        case pathtype
+        when 'overlayfs:'
           # Split on colon (:), ignoring any colon escaped by an escape character ( \ )
           # Pays attention to when the escape character is itself escaped.
-          fs_type, master_path, overlay_path = config_entry.split(/(?<!\\)(?:\\\\)*:/)
+          _, overlay_path = config_entry.split(/(?<!\\)(?:\\\\)*:/)
           if overlay_path
             Pathname.new(overlay_path)
           else
             # Malformed: fall back to prior behaviour
-            Pathname.new(config_entry)
+            Pathname.new(path)
           end
         else
-          Pathname.new(config_entry)
+          Pathname.new(path)
         end
       end