link monitor: replace events() with optionally continuous iteration

The way to use the monitor now is:
- Iteration on a stopped monitor:

    mon = Monitor()
    mon.start()
    mon.stop()
    for event in mon:
        do things with the event

- Iteration on a running monitor:

    mon = Monitor()
    mon.start()
    for event in mon:
        do things with the event

This is the first step towards having a thread that keeps netinfo
updated by events.

Change-Id: Ib8ca850fc324a5b4c0268541fd4d4d062706a159
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21430
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>