ssh-generator: suppress error message for vsock EADDRNOTAVAIL

ssh-generator: suppress error message for vsock EADDRNOTAVAIL

In logs in the Fedora OpenQA CI:
Nov 17 22:20:06 fedora systemd-ssh-generator[4117]: Failed to query local AF_VSOCK CID: Cannot assign requested address
Nov 17 22:20:06 fedora (generato[4088]: /usr/lib/systemd/system-generators/systemd-ssh-generator failed with exit status 1.
Nov 17 22:20:06 fedora systemd[1]: sshd-vsock.socket: Unit configuration changed while unit was running, and no socket file descriptors are open. Unit not functional until restarted.

AF_VSOCK is not configured there and systemd-ssh-generator should just exit
quietly. vsock_get_local_cid() already does some logging at debug level, so we
don't need to.

There is also a second bug, we report modifications to the unit have just
created. I think we have an issue open for this somewhere, but cannot find it.