PCI MSI: Replace 'type' with 'is_msix'

By changing from a 5-bit field to a 1-bit field, we free up some bits
that can be used by a later patch.  Also rearrange the fields for better
packing on 64-bit platforms (reducing the size of msi_desc from 72 bytes
to 64 bytes).

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>