side-effect of registering them, config_detach(9) cleared DVF_ACTIVE
before it called wddetach() or sddetach(). Even though sd(4)'s
detachment may have subsequently failed with EBUSY, we could not
begin new disk I/O on sd(4) because the device had been deactivated.
By analogy to sdstrategy(), test device_is_active() in wdstrategy()
and if it is false set b_errno to EIO instead of initiating new
disk I/O.
XXX We should decline more politely to start new I/O, since
XXX !device_is_active() may mean simply that the device is suspended.
XXX I suppose that EIO is safe as long as system suspension is
XXX all-or-nothing.
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