This fixes up, improves, and enables by default the use of LVM as
infrastructure.  Also removes some old copied LVM code from libvnode_xen.

The Docker storage backend can still be aufs; the aufs data just gets
stored in a massive LV if so.  Or the storage backend can also be
devicemapper+LVM-thin-provisioning (the new default).

See clientside/tmcc/linux/docker/README.md in this commit for more
detail, esp on LV sizing.