# apt-get install -t jessie-backports virtualbox-guest-x11
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
virtualbox-guest-x11 : Depends: xserver-xorg-legacy but it is not installable
E: Unable to correct problems, you have held broken packages.
# apt-cache policy xserver-xorg-legacy
In fact, the package xserver-xorg-legacy doesn't exist in jessie[-backports] so virtualbox-guest-x11 should not depend on it.