Implement block device live resize as a PUT /actions

[alxiord]

Currently, to resize a block device while the guest is running, a PUT must be issued to /drives, with the ID of the block device in question, and the same parameters used when it was created. This PUT does not recreate the resource, but instead triggers a rescan of the device. This goes against RESTful practices and complicates the code that handles PUT requests. The solution is to trigger the rescan from a PUT to /operations instead, in which case the resource would be the operation and not the drive.