virtualbox: wait for virtualbox guest additions installation to finish

Don't report that virtualbox guest additions failed to install unless
the install procedure has actually finished.
virtualbox: use ifup instead of /etc/init.d/networking

Due to ubuntu bug #1301015, /etc/init.d/networking does not work in 14.04.
This seems to be a new "feature" of ubuntu.

https://bugs.launchpad.net/ubuntu/+source/ifupdown/+bug/1301015

Use ifup to bring up new interfaces instead.
virtualbox: don't lose credentials in virtualbox images

Images that are imported from the running virtualbox daemon don't have
their defaultCredentials set.
virtualbox: bump version to 4.2.24

Guest-additions version 4.2.8 fail when the system lacks X support (like ubuntu-server).
Version 4.2.24 no longer reports this as an error.