Add support for os-assisted-volume-snapshots

This patch adds support for the assisted volume snapshots API extension.
This is used by Cinder to ask Nova to perform a volume snapshot on its
behalf.  It's required when the volume is actually file backed (like
qcow2) and the hypervisor needs to be involved in the snapshot
operation.

Required for blueprint qemu-assisted-snapshots

Change-Id: I50ee9bf92c8de98528638d1724fe35e07bed729e
Merge "Add support for os-assisted-volume-snapshots"