[SCSI] target: Convert rd_build_device_space() to use errno

This patch converts rd_build_device_space() to return errno usage
for failures in rd_create_virtdevice().

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>