Rewrite MSI-HOWTO

I didn't find the previous version very useful, so I rewrote it.
Signed-off-by: NMatthew Wilcox <willy@linux.intel.com>
Reviewed-by: NRandy Dunlap <randy.dunlap@oracle.com>
Reviewed-by: NGrant Grundler <grundler@parisc-linunx.org>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>