KVM/Fast Models howto wiki pages updated

I've gone through the documentation on the wiki about how to get going with KVM on Fast Models, to clean it up and reorganise it and add some of the missing bits (notably how to set up a KVM guest kernel and filesystem). It's now at: https://wiki.linaro.org/PeterMaydell/KVM/HowTo

The only minor point I'd still like to address is that at the moment we document the old-style "build your kernel and arguments into an .axf file" boot-wrapper, because my changes to support specifying them at model runtime haven't yet gone into the boot-wrapper git repo. When they do land I'll update the wiki.

(Yes, technically TCWG2011-A15-KVM says "one page summary" but I thought splitting it into four pages was much clearer :-))

-- PMM