msix: Note endian TODO item

MSIX, like PCI, is little endian.  Specifying native is wrong here,
but we need to check the rest of the file to determine if it's
as simple as flipping this macro.

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