I couldn't make the only -x11 package installable, because of too strict guest-dkms dependencies (they need the same version as the x11 package).
So instead of fixing virtualbox everywhere, I'll just provide the guest-* complete stack for lts-utopic.
virtualbox-lts-vivid - 4.3.34-dfsg-1+deb8u1ubuntu1.15.04.1~14.04.1ubuntu5
should be fine.
I couldn't make the only -x11 package installable, because of too strict guest-dkms dependencies (they need the same version as the x11 package).
So instead of fixing virtualbox everywhere, I'll just provide the guest-* complete stack for lts-utopic.
virtualbox- lts-vivid - 4.3.34- dfsg-1+ deb8u1ubuntu1. 15.04.1~ 14.04.1ubuntu5
should be fine.