Virtualbox-libre new version was changed completely, removing Oracle VM VirtualBox Extension Pack support and including a modified list with free distros presets endorsed by the Free Software Foundation for the virtual machine creation wizard. Non-free distros and others OS presets were removed.
It could generate problems with virtual machines created on Virtualbox from others distros or operating systems.
To solve it, you should change OSType parameter into your file.vbox manually.
example 1: Machine uuid="{9aaa999a-a99a-9999-aa9a-9a99aa999999}" name="My custom machine" OSType="Parabola_64" snapshotFolder="Snapshots" lastStateChange="2012-05-28T01:39:48Z"
example 2: Machine uuid="{9aaa999a-a99a-9999-aa9a-9a99aa999999}" name="My custom machine 2" OSType="Other" snapshotFolder="Snapshots" lastStateChange="2012-05-28T01:39:48Z"
OSType list:
1) Other
2) GNU
3) GNU_64
4) GNUHurd
5) GNULinux
6) GNULinux_64
7) Parabola
8) Parabola_64
9) gNewSense
10) gNewSense_64
11) Blag
12) Blag_64
13) Dragora
14) Dragora_64
15) Ututo
16) Ututo_64
17) Musix
18) Musix_64
19) Trisquel
20) Trisquel_64
21) Dynebolic
22) Venenux