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>