Extract base class from SingleFileEventHandler

The base class can contain code useful to other inotify users.
As it is “SingleFileEventHandler” can not be used in ganeti-rapi,
therefore it'll use its own small inotify handler class based
on this base class.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>