This reduces the dependencies pulled into default installations. (Closes: #901940)

We agreed to NOT make vbox<->xen mutually exclusive.
But having them in separate packages still at least gives people
affected by the following a chance to resolve it:
- https://askubuntu.com/questions/1060001/problems-starting-libvirt-daemon-in-ubunti-18-04
- https://bugzilla.redhat.com/show_bug.cgi?id=1278847
- https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1784276