Some targets may perform io of their own volition, eg. a mirror
performing recovery, a cache target pulling in different chunks.  We
cannot let them perform this io while the device is suspended.  This
patch adds 2 new methods to the target type, which instruct the target
to suspend/resume itself.  All targets start in the suspended state,
so should expect an initial resume call.  Simple targets do not need
to implement these functions.