core: allowed to add network label even if Vm using this network is
running.

Network label was checked to be unchanged in method
'onlyPermittedFieldsChanged'. This is extracted out into new method
'allowedNetworkLabelManipulation' which returns true if network label
was not changed, or if former label was not specified (null) and new
one is specified (!=null).

Change-Id: I57a658a4e2d4648917a265493cde4749ad15e3ad
Bug-Url: https://bugzilla.redhat.com/1134009
Signed-off-by: Martin Mucha <mmucha@redhat.com>