imgedit: fix hang in case qemu terminates before nbd client connects

To make it work self._closed needs to be initialized early because
even though exception is thrown from __init__(), __exit__() still will
be called and that one expects self._closed to be defined.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

scripts/imgedit.py

@@ -52,6 +52,7 @@ class nbd_file(object):
         self._filename = filename
         self._offset = 0
         self._buf    = None
+        self._closed = True
         self._process = subprocess.Popen("qemu-nbd %s" % filename,
                                         shell = True, stdout=_devnull)
         # wait for qemu-nbd to start: this thing doesn't tell anything on stdout
@@ -60,6 +61,8 @@ class nbd_file(object):
                 self._client = nbd_client("localhost")
                 break
             except:
+                if self._process.poll() != None:
+                    raise Exception('Qemu terminated with exit code %d' % self._process.returncode)
                 time.sleep(0.1)
         self._closed = False