Create symlink from LVM path to qcow2 file when launching

Regardless if nova is configured to use LVM cobalt will always use
qcow2 files when launching an instance. However, we still need
something at the LVM path location because nova will reconfigure
things on reboots. To handle this cobalt will create a symlink at
the LVM path location that targets the qcow2 root disk.

Change-Id: I41411d27d4dea2de5d79be4a66b8ca9286ce5335