virtio: fix feature bit checks

Several places check against the feature bit number instead of against
the feature bit. Fix them.

Cc: qemu-stable@nongnu.org
Reported-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
