scsi-disk: restruct emulation: core + TEST_UNIT_READY.

scsi-disk: restruct emulation: core + TEST_UNIT_READY.

Add new scsi_disk_emulate_command() function, which will -- when
finished -- handle all scsi disk command emulation except actual I/O
(READ+WRITE commands) which goes to the block layer.  The function
builds on top of the new SCSIRequest struct.

SCSI command emulation code is moved over from scsi_send_command() in
steps to ease review and make it easier to pin down regressions (if any)
using bisect.  This patch moves over TEST_UNIT_READY only.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>