This is still (or again?) broken in Ubuntu Xenial:
# dpkg -l | grep virtualbox
ii virtualbox 5.0.14-dfsg-2build1 amd64 x86 virtualization solution - base binaries
ii virtualbox-dkms 5.0.14-dfsg-2build1 all x86 virtualization solution - kernel module sources for dkms
ii virtualbox-ext-pack 5.0.16-1 all extra capabilities for VirtualBox, downloader.
ii virtualbox-guest-additions-iso 5.0.16-1 all guest additions iso image for VirtualBox
ii virtualbox-qt 5.0.14-dfsg-2build1 amd64 x86 virtualization solution - Qt based user interface
This is still (or again?) broken in Ubuntu Xenial:
# dpkg -l | grep virtualbox guest-additions -iso 5.0.16-1 all guest additions iso image for VirtualBox
ii virtualbox 5.0.14-dfsg-2build1 amd64 x86 virtualization solution - base binaries
ii virtualbox-dkms 5.0.14-dfsg-2build1 all x86 virtualization solution - kernel module sources for dkms
ii virtualbox-ext-pack 5.0.16-1 all extra capabilities for VirtualBox, downloader.
ii virtualbox-
ii virtualbox-qt 5.0.14-dfsg-2build1 amd64 x86 virtualization solution - Qt based user interface
=> virtualbox at 5.0.14, extpack at 5.0.16.