Refactor -drive option to better reflect separation of backend and frontend.
To create a separate drive backend, today we need to specify:
-drive file=/path/to/image,if=none,id=foo -device virtio-blk-pci,drive=foo
This is awkward to use at best and is not symmetric to other options in qemu. This feature proposes a new syntax:
-blkdev file=/path/to/image,id=foo -device virtio-vlk-pci,blkdev=foo
-blkdev only takes valid backend options. Like netdev and fsdev, the same name is used to connect the backend. -drive would continue to be supported as a syntactic short-cut.