plugin: wait until the label is set

After reseting the label to an empty string, wait until a query confirms
it. This is to account for the delay required for a modification to show
in the front ends.

Change-Id: Ib98bd6fb9d6ae97d1028d56afb4d855dec5c19c6
Signed-off-by: Loic Dachary <loic@dachary.org>