Unmanaged devices are attached to guests in two steps: first,
the device is detached from the host and marked as inactive;
subsequently, it is marked as active and attached to the guest.

If the daemon is restarted between these two operations, we lose
track of the inactive device.

Steps 5 and 6 of virHostdevPreparePCIDevices() already subtly
take care of this situation, but some planned changes will make
it so that's no longer the case. Plus, explicit is always better
than implicit.