vhost-user-blk device (basic support)

[kalyazin]

Feature Request

Add support for vhost-user-blk devices by leaving snapshotting support for vhost-user-blk out of scope.

Describe the desired solution

Firecracker implements a vhost-user-blk frontend.

Describe possible alternatives

Existing virtio block device.

Additional context

NA

Checks

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