whitelist host virtio networking features

This patch is a followup to 8eca6b1bc770982595db2f7207c65051572436cb,
fixing crashes when guests with 2.6.25 virtio drivers have saturated
virtio network connections.

https://bugs.edge.launchpad.net/ubuntu/+source/qemu-kvm/+bug/458521

That patch should have been whitelisting *_HOST_* rather than the the
*_GUEST_* features.

I tested this by running an Ubuntu 8.04 Hardy guest (2.6.24 kernel +
2.6.25-virtio driver).  I saturated both the incoming, and outgoing
network connection with nc, seeing sustained 6MB/s up and 6MB/s down
bitrates for ~20 minutes.  Previously, this crashed immediately.  Now,
the guest does not crash and maintains network connectivity throughout
the test.

Signed-off-by: Dustin Kirkland <kirkland@canonical.com>
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
