An exception fix, a python versioning fix, and some better color choices for dark themes

Hi,

4 commits that I would like you to merge. 

0815726: Just change the shebang to specify python2 instead of just python. I dont think this is controversial, but it does mean that ./openxenmanager will run straight away on distros where python3 is the default:

6b44dc9: I got exceptions for VMs where xen doesnt know about os version, i.e. asking for os_version would give an exception, which would cause other exceptions.

The other two commits: I'm running Adwaita Dark and Xfce-Dusk, which are both dark themes, and some of the colors in oxm are illegible. So I changed the hardcoded colors in oxc.glade to use the "names" instead. So, for example the #5eb352d952d9 color is replaced by "darkblue". At least my themes handle this better.
 
Get in touch if there's anything that is unclear!
/Lars