LiveMerge: fix delete while iterating

A coding mistake slipped into the onBlockJobEvent handler.  We are
iterating a dictionary while deleting items.  We need to iterate over
the keys in this case, not the k,v pairs.

Change-Id: I6074c58c4c9aadd60cfe2ef813d46d70ecab3eae
Signed-off-by: Adam Litke <alitke@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28863
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>