core: use last successful proxy in wait-for-status

Power Management proxy selection keep selecting the same proxy first
even is it constantly fails

This patch changes the waiting for status after stop/start PM action to
use the last successful proxy along the wait-for-status loop instead of
searching for proxy each time that might be resulted in significant
delays in case that the preferred proxy (according to the proxy selection
settings_ has some connectivity issues and can not access the PM card.

Change-Id: I7684826be6098e952134366ce480ddb1ca3a2c99
Signed-off-by: Eli Mesika <emesika@redhat.com>