When multiple devices share resources and one of them is to be passed
through to a guest, security of the entire system and of respective
guests individually cannot really be guaranteed without knowing
internals of any of the involved guests.  Therefore such a configuration
cannot really be security-supported, yet making that explicit was so far
missing.

Resources the sharing of which is known to be problematic include, but
are not limited to
- - PCI Base Address Registers (BARs) of multiple devices mapping to the
  same page (4k on x86),
- - INTx lines.