utils/ostree: deployment path helper

Add a simple helper method that returns the path for a deployment,
given the sysroot, the osname, the reference or commit and the
deployment serial. Path might not exist.
`

osbuild/util/ostree.py

@@ -125,3 +125,17 @@ def rev_parse(repo: PathLike, ref: str) -> str:
         raise RuntimeError(msg)
 
     return msg
+
+
+def deployment_path(root: PathLike, osname: str, ref: str, serial: int):
+    """Return the path to a deployment given the parameters"""
+
+    base = os.path.join(root, "ostree")
+
+    repo = os.path.join(base, "repo")
+    stateroot = os.path.join(base, "deploy", osname)
+
+    commit = rev_parse(repo, ref)
+    sysroot = f"{stateroot}/deploy/{commit}.{serial}"
+
+    return sysroot