virtio@pci: Fix assertion on detach.

If the child never attached in the first place, it's OK for it to not
have detached.

XXX This should not be a set of flags; this should be a state
enumeration, because some flags make no sense, like FINISHED|FAILED.

XXX This should not be asserted separately in each bus; there should
be a single place in virtio.c to assert this, uniformly in all buses.

PR kern/57357

XXX pullup-10

(riastradh)