Enhancement virtio-scsi bus support for volume

VirtIO SCSI is a new para-virtualized SCSI controller device for KVM instances.It has been designed to replace virtio-blk, increase it's performance and improve scalability. The interface is capable of handling multiple block devices per virtual SCSI adapter, keeps the standard scsi device naming in the guests (e.x /dev/sda) and support SCSI devices passthrough.

Currently, virtio-scsi bus has been supported when booting from glance image with property "hw_scsi_mode=virtio-scsi" or cinder volume created by this type of image, which is implemented by BP ([1]) in Icehouse.

However, for volumes which were not created from image, we have no method to specify using virtio-scsi controller for it.

The aim of this BP as follows:

For bootable volumes which including virito-scsi driver, user can set specific metadata for them. When booting from these volumes with "scsi" bus type, use virtio-scsi controller instead of the default lsi controller.

The main use case is to improve performance in I/O-intensive applications.

[1] https://blueprints.launchpad.net/nova/+spec/libvirt-virtio-scsi-driver