Make Ignite use containerd's `devmapper` snapshotter

[twelho]

containerd has (re)gained a device mapper snapshotter, courtesy of the Firecracker team for their firecracker-containerd project. This allows containerd to store the image/container layers (snapshots) as block devices managed by device mapper. Internally the solution uses the thin target to create a thin-provisioning pool, and the snapshot target for the layering.

This poses an interesting approach to VM image/layer handling for Ignite, the top-layer block devices emitted by the snapshotter should be directly passed to Firecracker which avoids the current loop mounts and need to export the image to an ext4-formatted file altogether.