Update SolidFire to use target driver model

Update SolidFire to use target driver model

A while back we broke the inheritance chain of transport and backend
by moving the target drivers outside of the drivers.

This means you can instantiate and use any target (data transport)
method that your device supports without requiring a separate
driver (again, if your device can support it).  This was
particularly important for the LVM ref driver and the 5 iSCSI
options that go with it.

This patch converts the SolidFire driver to use the same model.  For
the most part it's pretty simple:
* Create an internal class for the target driver
* Instantiate the target driver that you want to use
* wrap the old transport calls that come to the driver to call
  the new target driver objects

This exposed some issues with the abc work that was being done.
There are a number of items that are marked as required but are
really only necessary for LVM based drivers.  So this also
creates a san/3'rd party iSCSI subclass as well that has the
more appropriate abc settings for such devices.

Change-Id: I02ceaac58d0e4176ccad423763ae0101cfb8446b