Enable nbd if you're running an lxc virt_type without an lvm backend

If nbd isn't enabled you can't boot instances with libvirt using lxc
(unless you're using an lvm backend).

Closes-Bug: #1487195

Co-Authored-By: Andrew Melton <andrew.melton@rackspace.com>

Change-Id: I08c4d498ed35166f566291d9530ca1ecfae05625

lib/nova

@@ -354,6 +354,12 @@ function configure_nova {
                             sudo mount /cgroup
                         fi
                     fi
+
+                    # enable nbd for lxc unless you're using an lvm backend
+                    # otherwise you can't boot instances
+                    if [[ "$NOVA_BACKEND" != "LVM" ]]; then
+                        sudo modprobe nbd
+                    fi
                 fi
             fi
         fi