Adding a PCIe card can renumber the buses behind it, so a hostpci entry
that names a PCI address can pass through the wrong device. These start
the VM only after finding each device by vendor/device/subsystem ID or
by its MAC-pinned interface, repoint its resource mapping if it moved,
and refuse to start on anything ambiguous. Its dependents then wait for
a port on it to answer before starting, so they find their shares up.
