This adds support to LinuxContainer to be able to be provided an
optional writable layer. This is useful as today the rootfs size
is determined by the size of the image block itself which is determined
at unpack time of the image. This leaves a lot to be desired because
a user might want a larger or smaller rootfs size, but it's not configurable
on a per container basis because of this. To support this, we can pass
an additional writable block device that we can overlayfs with the image
contents in the guest. All writes will go to this writable layer, and
the rootfs size in the container is now whatever size this writable layer
is.

The block devices (via our ext4 package, but you could use whatever) are
cheap enough to generate on the fly that you can choose whatever size you
want easily.