vfio-pci: Add support for MSI affinity

vfio-pci: Add support for MSI affinity

When MSI is accelerated through KVM the vectors are only programmed
when the guest first enables MSI support.  Subsequent writes to the
vector address or data fields are ignored.  Unfortunately that means
we're ignore updates done to adjust SMP affinity of the vectors.
MSI SMP affinity already works in non-KVM mode because the address
and data fields are read from their backing store on each interrupt.

This patch stores the MSIMessage programmed into KVM so that we can
determine when changes are made and update the routes.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>