merge: reflect parent device manage/unmanage state for VLANs (rh #1114681)

VLAN device watches the parent and reacts to its state change. However, on
creation, the device was made managed, even if the parent was unmanaged. Fix
that and export VLAN parent device property too. The property is added to
libnm/libnm-glib and nmcli as well.

https://bugzilla.redhat.com/show_bug.cgi?id=1114681