virsh: Allow attach-disk to specify disk target bus

virsh: Allow attach-disk to specify disk target bus

By default, the bus type is inferred from the style of the device
name('target' in this command), e.g. a device named 'sda' will
typically be exported using a SCSI bus. Actually, not only SCSI bus,
but USB/SATA bus also use this kind of device name. So add '--bus'
option for attach-disk command to allow user specify the target bus.

Signed-off-by: Yanbing Du <ydu@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>