attach_device: Support more device target bus for disk attach

1. libvirt support virtio, usb and scsi device target bus for disk attach,
so move class VirtIODiskBasic to VirtualDiskBasic to support target bus
specified in cfg.
2. Add two positive cases for usb and scsi disk attach and two negative
cases.

Signed-off-by: Yanbing Du <ydu@redhat.com>