select bus type of virtual disk when launching instance

When launching an VM instance, user can not specify an applicable bus type for virtual disk. Actually the bus type will be determined by device type (disk,cdrom) and hypervisor (kvm/qemu, lxc,xen,uml) combinations in get_disk_bus_for_device_type(virt_type,image_meta,device_type="disk"), or by hypervisor and device prefix combinations in get_disk_bus_for_disk_dev(virt_type,disk_dev).

e.g. The default bus type for kvm/qemu is virtio which is paravirtualized IO and requires guest os to have the driver. But it is not always true for some type of guest OS such as Windows 2003. It will block the guest OS installation since the virtio driver is not there. If we can specify the bus type to a general one such as IDE, the guest OS installation can be moved forward. Once the installation done, the driver can be installed in guest OS to resolve this problem.

Use Case:
User want to create a new image in the following steps:
1. install a fresh guest OS (Windows 2003) through an ISO image;
2. snapshot the instance;
3. convert the snapshot to a new image.