core: Changed Bond constructor for backwards compatibility

Before the Bond subclass was implemented, the old way to check whether a
VdsNetworkInterface was to check the value of its "bonded" field. This
is now preserved, so that any Bond object would still comply with older
code.

Change-Id: I93d970df3fe1e22e8116d0b5683e82851037f6c8
Signed-off-by: Lior Vernia <lvernia@redhat.com>