partially handle disks in file system plugin

* open/read/write/move/close/unlink
* remote read/write operations not supported yet
* update the s4u-io-file-system example
* To be used with this plugin, a <disk> has to declare some <props>
   * size (with unit) [mandatory]
   * mount            [mandatory]
   * content          [optional]
* TODO
   * write some documentation
   * add some checks/asserts if user tries to
      * use plugin with no <prop> declared
      * mix disk and storage