manager: ensure auto default connection is deleted when a veth goes away

When the link goes away the manager keeps software devices alive as unrealized because there is still a connection for them.

If the device is software and has a NM-generated connection, keeping the device alive means that also the generated connection stays alive. The result is that both stick around forever even if there is no longer a kernel link.

Add a check to avoid this situation.

https://bugzilla.redhat.com/show_bug.cgi?id=1945282

Fixes: cd0cf922 ('veth: add support to configure veth interfaces')