slow start issue with btrfs enabled lkl

[thehajime]

reported by @petrosagg
#296 (comment)

[tavip]

@thehajime @petrosagg I think this is an upstream issue, and it should be fixed upstream just like the virtio issue fixed by @liuyuan10. We can keep this issue here, just to track it, but I don't think we should patch LKL.

[petrosagg]

@tavip that makes sense. We can still avoid the XOR tests though by defining XOR_SELECT_TEMPLATE just like the x86 arch

[Rondom]

@petrosagg It may be a bit challenging to accomplish in a maintainable fashion, but we should try to choose the method depending on the arch IMO.

[tavip]

@petrosagg it looks like regs8 is a good choice for both 32 and 64bits. I will submit a PR soon with that choice and we can refine it later, per arch, if we need.