assemblers/qemu: fix the partition UUID in the pipeline

Otherwise, sfdik would pick one at random. We want our images to be
reproducible to the extent possible, so we must move all randomness
out of the assemblers when we can.

Signed-off-by: Tom Gundersen <teg@jklm.no>

samples/base-qcow2.json

@@ -72,6 +72,7 @@
       "options": {
         "format": "qcow2",
         "filename": "base.qcow2",
+        "ptuuid": "0x14fc63d2",
         "root_fs_uuid": "76a22bf4-f153-4541-b6c7-0332c0dfaeac",
         "size": 3221225472
       }