Comment 5 for bug 1628336

Somehow looks like the creation of the nbd device does not reliably trigger the partition scanning in kernel. The following work-around would at least here get past the problem:

--- /usr/bin/mount-image-callback 2016-09-01 22:07:51.000000000 +0200
+++ mount-image-callback 2016-09-28 11:46:29.404822620 +0200
@@ -321,6 +321,7 @@
     debug 1 "unpartitioned disk."
    else
     debug 1 "partitioned disk."
+ blockdev --rereadpt $nbd
    fi
    ptnum=$nptnum
   fi