Virtualbox low resolution mode
You have to change the info.plist with admin rights. For example in terminal type:
sudo nano /Applications/VirtualBox.app/Contents/Resources/VirtualBoxVM.app/Contents/Info.plist
Then you can change the following key to false:
<key>NSHighResolutionCapable</key> <true/>
should now be :
<key>NSHighResolutionCapable</key> <false/>