scsi-block: fix direction of BYTCHK test for VERIFY commands

The direction is wrong; scsi_block_is_passthrough returns
false for commands that *can* use sglists.

Reported-by: Zhang Qian <zhangqian@sangfor.com.cn>
Fixes: 8fdc7839e40f43a426bc7e858cf1dbfe315a3804
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
