core: Check if balloon device is present in stalled balloon check

Check for stalled balloon (inflated but uncontrolable) accesses
balloon data. That might cause Null pointer exception when
the device is not there or we do not have all needed information.

Change-Id: Ie9074fc67913e3c5ad9ad81744bfe4de27a19636
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1035346
Signed-off-by: Martin Sivak <msivak@redhat.com>