test.volume_application: Fix installation problems.

 * As this line in unattended_install module:
   [vm = env.get_vm(params["main_vm"])]
   It needs 'main_vm' and a VM object for a new installation.
   So new vm should be registered to env.
 * Unattended_install sets kickstart ISO to /dev/sr0 as default.
   So changing the cdroms order to fit this.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>