tests.volume_application: Fix installation.

After having checked with a pure virt-test/tp-libvirt tree, these problems were fixed:
1. Process problems for new installation with changed main_vm.
2. Run installation and fix some 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.
```