assemblers/qcow2: Pass size explicitly

Don't try to guess how much room the filesystem will take up. In
practice, most people will want to specify a size anyway, depending on
their use case.

As is typical for osbuild, there are no convenience features for the
pipeline (it's not meant to be written manually). `size` must be given
in bytes and it must be a multiple of 512.

samples/base-qcow2.json

@@ -67,8 +67,9 @@
     {
       "name": "org.osbuild.qcow2",
       "options": {
-	"filename": "base.qcow2",
-        "root_fs_uuid": "76a22bf4-f153-4541-b6c7-0332c0dfaeac"
+        "filename": "base.qcow2",
+        "root_fs_uuid": "76a22bf4-f153-4541-b6c7-0332c0dfaeac",
+        "size": 3221225472
       }
     }
 }