core:Class cast exception when fence fails

The problem was that in the case that the proxy host is blocked by
iptables to access the PM card we got a message result in the RPC XML
return value which is Object[].
In the code, there was attempt to cast this result to a string which
obviously failed with ClassCastException.
This patch adds check for the message type and gets it properly as
string in the case that we got Object[].

Change-Id: I511599ce3c6dbf313cbe8d1f0f57e329a4aa1d79
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1114977
Signed-off-by: Eli Mesika <emesika@redhat.com>