target: Add missing XCOPY I/O operation sense_buffer

This patch adds the missing xcopy_pt_cmd->sense_buffer[] required for
correctly handling CHECK_CONDITION exceptions within the locally
generated XCOPY I/O path.

Also update target_xcopy_read_source() + target_xcopy_setup_pt_cmd()
to pass this buffer into transport_init_se_cmd() to correctly setup
se_cmd->sense_buffer.

Reported-by: Thomas Glanzmann <thomas@glanzmann.de>
Reported-by: Douglas Gilbert <dgilbert@interlog.com>
Cc: Thomas Glanzmann <thomas@glanzmann.de>
Cc: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>