virtio-scsi: Fix num_queue input validation

We need to count the ctrlq and eventq, and also cleanup before
returning. Besides, the format string should be unsigned.

The number could never be less than zero.

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