Don't check for bus master for old guests

Older Linux guests don't activate the bus master enable bit. So for those we
can just try to be clever and track if they set the DEVICE_OK bit even though
bus mastering is still disabled.

Under that condition we can disable the windows safety check. With that logic
in place both guests should work just fine. Without PCI hotplug breaks
virtio-net in Linux < 2.6.34 guests.

Signed-off-by: Alexander Graf <agraf@suse.de>
CC: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>