New struct for Libvirt/tests

    Make a dir struct for libvirt-tests. Referring to "virsh help",
    the virsh_cmd module contains 11 parts. The outline looks
    like:
    
    libvirt/tests/                              virsh_cmd/
                |---cfg/                            |---domain/
                    |---kernel_panic.cfg      |---monitor/
                    |---remove_guest.cfg    |---host/
                    |---virsh_cmd/              |---interface/
                |---src/                            |---filter/
                    |---kernel_panic.py       |---network/
                    |---remove_guest.py     |---nodedev/
                    |---virsh_cmd/              |---secret/
                                                      |---snapshot/
                                                     |---pool/
                                                     |---volume/

And fix a small typo of virsh_attach_detach_disk.cfg.