If you want to pass through a raw disk device, you don’t create a storage pool. Storage pools store images.
In the storage selection step, select “Select or create custom storage”, then type /dev/sda in the entry field. Click Forward.
Actually, it’s better if you don’t use the /dev/sd* entries for this, use the link to the device in the /dev/disk/by-id directory. This way, if the name of the device changes (because you plugged in a usb drive or whatever), it’ll still work.