[Staging] vhost: fix vhost force with msix=off

In response to a bug report on IRC: this should fix it I think?
Need to test properly but out of time for today,
compiled only. Hope this helps.

http://lists.gnu.org/archive/html/qemu-devel/2013-04/msg02809.html
(cherry picked from commit 18fbb339e3218ab8d7018aadc815a8f2f5104e7c)