Fix oops on 2.6.25 guest (Rusty Russell)

I believe this is behind the following:
https://bugs.edge.launchpad.net/ubuntu/jaunty/+source/linux/+bug/331128

virtio_pci in 2.6.25 didn't do feature negotiation correctly: it acked every
bit.  Fortunately, we can detect this.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6975 c046a42c-6fe2-441c-8c8c-71466251a162
