vdsm: Do not fail releasing a VM due to agent stopping errors

If the guest agent socket couldn't get created for whatever reason
VDSM will no longer be able to stop release the VM without killing
it manually and restarting the VDSM service.
This patch is fixing this problem and also logs the occurrences.

Change-Id: I10ac23b11294dc90e4980944d7ba6fad6abc22dc
Bug-Url: https://bugzilla.redhat.com/985770
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17072
Reviewed-by: Dan Kenigsberg <danken@redhat.com>