devel branch lacks virtualbox-guest-x11
Without this package, both 32-bit and 64-bit guests work for me, but display is limited to 1024x768.
Install virtualbox-* 5.1.14-dfsg-3~bpo8+1 from our custom APT repository (refs: #12307).
Revert "Don't install virtualbox-guest-x11 (refs: #12307)."
This reverts commit 1fe3bb4bbcefbc2308b716247b2971f3b8b7fb4f.