scsi: Support TEST UNIT READY in the dummy LUN0

scsi: Support TEST UNIT READY in the dummy LUN0

SeaBIOS waits for LUN0 to respond to the TEST UNIT READY command
in order to decide whether it should part of the boot sequence.
If LUN0 does not respond to the command, boot is delayed by up
to 5 seconds.  This currently happens when there is no LUN0 on
a target.  Fix that by adding a trivial implementation of the
command.

Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 1cb27d9233d572826b45bd8498d2fab1b6f01df9)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>