target: move code for CDB emulation

Move the existing code in target_core_cdb.c into the files for the command
sets that the emulations implement.

(roland + nab: Squash patch: Fix range calculation in WRITE SAME emulation
 when num blocks == 0s)

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