host: First pass on an implementation of a "Volumes" system as described in #534.

Some interfaces are not yet complete, but documentation indicates the direction they're expected to go.  ZFS will be the leading implementation, with others expected to follow.

Signed-off-by: Eric Myhre <eric@flynn.io>
host: Split apart tests for snapshotting, and specifically test over alterations to both sides of a cloning operation.

Signed-off-by: Eric Myhre <eric@flynn.io>
host: When creating zpools, place their mounts under their own directory tree.

Signed-off-by: Eric Myhre <eric@flynn.io>
WIP

Signed-off-by: Eric Myhre <eric@flynn.io>