virsh_update-device: case expand and tiny bugs fix

1. Support different disk type: cdrom, floppy
2. Support different disk mode: none, readonly, shareable
3. Cover more sub test cases
4. Tiny bugs fix

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #181 from will-Do/update_device

virsh_update-device: case expand and tiny bugs fix