Restructure add_eltorito.
Restructure add_eltorito.

Basically make it so that it always goes through the
finish_add method so we don't forget any cleanup tasks.

Signed-off-by: Chris Lalancette <281cd07d7578d97c83271fbbf2faddb83ab3791c@gmail.com>

clalancette/pycdlib,clalancette/pyiso