agent: Send 'uninstalled' notification non blocking

The "echo" command blocks if the host is not connected to the
virtio-serial port. Replace it with a short python script that
opens the port with the O_NONBLOCK flag. This way, removing the
agent's doesn't hung if vdsm (or any other listener) doesn't
exist during the removal.

Change-Id: I5b49e53cda2f21bd759b0f3f67619f7f1e45688f
Signed-off-by: Gal Hammer <ghammer@redhat.com>
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>