Add a mode to use a dummy Wii NAND.

Eventually, netplay will be able to use the host's NAND, but this could
still be useful in some cases; for TAS it definitely makes sense to have
a way to avoid using any preexisting NAND.
Move NandPaths to Core to avoid layering violation introduced by previous commit (its paths depend on Core::g_want_determinism).

Not that there seems to be much layering to speak of, but if anything,
Common should be stuff that's not necessarily Dolphin specific and could
be used elsewhere.  NandPaths is not in that category, and it's only
used from Core.
Disable /dev/usb/hid too.