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>