core: changing error severity of illegal search

WARN message logged versus DEBUG or INFO message with rhevm-shell
provided bogus search

This patch changes the severity of illegal searches done by the user
from WARN to INFO

Change-Id: Ic6abcdc24e1538937cc317c2fc13a913cb542786
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1133553
Signed-off-by: Eli Mesika <emesika@redhat.com>