Check xen / vbox incompatibility

There is an issue that seems to hit people at very low frequency.
Reason is using xen (seldom) while also having virtualbox installed (seldom as well).

Together they both register a signal handler and break with an assert.
See: https://askubuntu.com/questions/1060001/problems-starting-libvirt-daemon-in-ubunti-18-04

This is long known upstream:
 - https://bugzilla.redhat.com/show_bug.cgi?id=1278847
 - https://www.redhat.com/archives/libvir-list/2015-November/msg00366.html

In a crash report or dump the backtrace will contain something like:
  libxl_childproc_setmode
  libxl__sigchld_needed
  sigchld_installhandler_core
  __GI___assert_fail
  __assert_fail_base
  libvirtd

It is a bit silly VirtualBox and Xen are not generally Mutually exclusive (althought you can only run one at a time), but they install along. Only in the context of this bug they are mutually exclusive for libvirt.

This is rather low on prio, but if we e.g. really will split connection drivers [1] we might add a conflicts between xen / virtualbox just on xen connection drivers.

Workaround until then:
Pick one VirtualBox or Xen libraries and deinstall the other.

[1]: https://salsa.debian.org/libvirt-team/libvirt/merge_requests/2