scsi-bus: fix transfer length and direction for VERIFY command

The amount of bytes to transfer depends on the BYTCHK field.
If any data is transferred, it is sent to the device.

Cc: qemu-stable@nongnu.org
Tested-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
