configNetwork: Improve the validation of netmask

Change-Id: I3c0a39f3f681a9e587f9118246e42a29e18d604b
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/7713
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>