That's an option for virtualization, needed for things like pci-e passthrough. Shouldn't affect the number of cards recognized in any way, if anything I'd actually disable it (and it's probably disabled by default).

If you can return it then your probably should. Although I know some people use am3 systems for mining, I personally got rid of my amd boards a long time ago — intel is just better for mining rigs (mostly because it's a more power efficient platform consuming less power, and there's also plenty of boards with 7* pci-e slots and 2* m2 slots, where you can connect up to 9 gpus without any splitters). So I wasn't following the amd boards for quite a while, and can't really suggest anything. If the only purpose of this rig is cryptomining then you might just sell your fx6300 and switch to intel. Either to something on z97 (if you just have to keep that ddr3 ram), or to z270 (will need go to ddr4).