msi: Add msi_get_message()

vfio-pci and pci-assign both do this on their own for setting up
direct MSI injection through KVM.  Provide a helper function for
this in MSI code.

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