osbuild: fix missing initialization of fd in osbuild.loop.Loop

When osbuild.loop.Loop calls `__init__()` it assigns the `self.fd`
on open. However if that open call fails for whatever reason
(not found, permissions) the cleanup in `__del__` will fail in
confusing ways because `self.fd` is not initialized yet. It
also prevents the correct error from getting reported. A tiny
test is added to ensure this does not regress.

osbuild/loop.py

@@ -120,6 +120,7 @@ class Loop:
         self.devname = f"loop{minor}"
         self.minor = minor
         self.on_close = None
+        self.fd = -1
 
         with contextlib.ExitStack() as stack:
             if not dir_fd: