Image-Backed Jails

[fabianfreyer]

This would probably end up being a UFS over ZVOLs

Open questions:

• Should we keep zvols in <base>/storage, or not. We probably have the following options:
  1. Extraction scheme would be similar to ZFS Support #3, but in zvols. Each layer would be its own zvol. We could quickly clone this into <base>/runtime when starting a jail.
  2. Copy files from dataset in <base>/storage into a new zvol in <base>/runtime on start. Copy files back on commit.
  3. Copy files from dataset in <base>/storage into a new zvol in <base>/runtime on first start. Clone that into <base>/runtime. Copy files back into dataset and/or clone zvol back into <base>/runtime on commit.