core: Block removing network when it's related to iSCSI bond.

Add validation which should block remove of network when it is related
to the iSCSI bond.

Change-Id: I68bd390175d42ee4e33773e12d6184a97a755eed
Bug-Url: https://bugzilla.redhat.com/1094196
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>