scsi-disk: fix VERIFY for scsi-block

When a scsi-disk object receives VERIFY command with BYTCHK bit being zero,
scsi_block_is_passthrough returns false and finally makes req being proceeded
by scsi_block_dma_command. Because scsi_block_dma_command has removed process
of VERIFY, QEMU will abort in this function.

Reported-by: Junlian Bell <zhongjun@sangfor.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>