scsi: fix searching for an empty id

The conditions for detecting no free target or LUN were wrong.

The LUN loop was followed by an "if" condition that is never
true, because the loop is exited as soon as lun becomes equal
to bus->info->max_lun, and never becomes greater than it.

The target loop had a wrong condition (<= instead of <).  Once
this is fixed, the loop would fail in the same way as the LUN
loop.

The fix is to see whether scsi_device_find returned the device with the
last (channel, target, LUN) pair, and fail if so.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
