target: consolidate backend attribute implementations

Provide a common sets of dev_attrib attributes for all devices using the
generic SPC/SBC parsers, and a second one with the minimal required read-only
attributes for passthrough devices.  The later is only used by pscsi for now,
but will be wired up for the full-passthrough TCMU use case as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>