pci: Error on PCI capability collisions

Nothing good can happen when we overlap capabilities. This may happen
when plugging in assigned devices or when devices models contain bugs.
Detect the overlap and report it.

Based on qemu-kvm commit by Alex Williamson.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Don Dutile <ddutile@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>