vhost-user-blk device (snapshot support)

# Feature Request

Add snapshot support for vhost-user-blk devices.

## Describe the desired solution

Firecracker implements saving and restoring from snapshot VMs that have block devices backed by vhost-user attached.

## Describe possible alternatives

The status quo is VMs having vhost-user-blk devices cannot be snapshotted. We can leave it as is.

## Additional context

[vhost-user-blk basic support](https://github.com/firecracker-microvm/firecracker/issues/4266)

## Checks

- [x] Have you searched the Firecracker Issues database for similar requests?
- [x] Have you read all the existing relevant Firecracker documentation?
- [x] Have you read and understood Firecracker's core tenets?