msix: Allow full specification of MSIX layout

Finally, complete the fully specified interface.  msix_add_config()
gets folded into msix_init() because we now have quite a few parameters
to pass and rolling it in let's us error earlier, avoiding the ugly
unwind exit path.  msix_mmio_setup() also gets rolled in, just because
it's redundant to rediscover offsets when we already have them for
such a tiny function.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>