Use correct ceph_args variable

Fixes: 6db6f80 ("Replace server XML with Nokogiri::XML::Builder")

Author: ekohl

lib/fog/libvirt/models/compute/server.rb

@@ -363,7 +363,7 @@ def to_xml
                         end
                       end
 
-                      xml.target(:dev => target_device, :bus => args["bus_type"] == "virtio" ? "virtio" : "scsi")
+                      xml.target(:dev => target_device, :bus => ceph_args["bus_type"] == "virtio" ? "virtio" : "scsi")
                     end
                   else
                     is_block = volume.path.start_with?("/dev/")