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>
attach_device: Fail early if attach-device command return value is not
expected

If test fail at attach-device step, it's unnecessary to run
preboot_action  and postboot_action steps.

Signed-off-by: Yanbing Du <ydu@redhat.com>
attach_device: Skip serial relate test if it not supported

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #224 from will-Do/attach_device

attach_device: Support more device target bus for disk attach