openvpn: prevent deletion if instance is assigned to an interface

[fichtner]

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• I have read the contributing guide lines at https://github.com/opnsense/core/blob/master/CONTRIBUTING.md
• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

Is your feature request related to a problem? Please describe.

When starting an OpenVPN instance from scratch and deleting the old one the probable interface assignment is never cleared which call cause all sorts of "strange" issues similar to when a hardware interface would disappear.

Describe the solution you like

When deleting an OpenVPN instance (server or client) make sure that the device is not being used in an interface. If that is the case block the deletion until the assignment was modified or the interface deleted.

Describe alternatives you considered

N/A

Additional context

https://forum.opnsense.org/index.php?topic=24248.0