PCI MSI: Add example request loop to MSI-HOWTO.txt

Encourage driver writers to think about supporting a variable number
of MSI-X interrupts, and give an example of how to do such a
request.

Acked-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>