manager: don't activate device if the parent is missing [bg/device-parent-rh1774074]
In multiple places we currently proceed to creating a virtual device even if the connection specifies a parent device which is missing. This can be easily reproduced with:
nmcli con add type vxlan ifname vxlan1
id 43 remote 172.25.1.1
which creates a vxlan1 interface without activating any connection. Add a check to prevent this.