Multiple Capability Sets Per Backend

Currently backends can only return one number for "free space" and a set of capabilities. If the backend actually manages a device with two pools of storage with different capabilities, there is no way to report the free space for those two pools other than to setup two whole backends managing the same device.

This proposal is to make a small tweak to the get_volume_stats() driver entry point to allow it return a list of dicts rather than a single dict. Existing driver can easily be updated to return singleton lists to achieve the exact same behavior that exists today. On the scheduler side, new logic will be needed to handle an additional level of indirection, so the scheduler can perform scheduling across the "pools" and after the right pool is selected, the request will be forwarded onto the backend associated with that pool.

This proposal was discussed in HK as part of my vendor-neutral-extra-specs talk. Obviously we won't be pursuing standardization of extra_specs any further, but this small proposal was not controversial at that time.