Following testing in more extreme situations, the following problems
turned up:

- The error handler can offline the device during DV (most particularly
true when transport parameters are undetectably mismatched).  Fixed by
modifying the state model to allow this and then having DV set the
device back online for the retry.

- DV needs to be serialised.  Fixed by introducing a per device
semaphore.

- Cosmetically, it's nice to trigger DV from userland, so added a
revalidate sysfs entry.