nixos/nixos-containers: fix interface name escaping in systemd.device unit name

[duament]

systemd escapes interface names in generated .device units. e.g. dummy-test -> dummy\x2dtest

When a non-alphanumeric interface (e.g. dummy-test) is set in containers.<name>.interfaces and autoStart is enabled, systemd will wait for a wrong device, resulting a 5 minutes delay to start the container. Logs:

subtest: Simple dummy interface is up, with autoStart enabled
autoStart: waiting for unit container@autoStart
autoStart: waiting for the VM to finish booting
autoStart: Guest shell says: b'Spawning backdoor root shell...\n'
autoStart: connected to guest root shell
autoStart: (connecting took 0.00 seconds)
autoStart: (finished: waiting for the VM to finish booting, in 0.00 seconds)
autoStart # [  312.936817] systemd[1]: sys-subsystem-net-devices-dummy-test.device: Job sys-subsystem-net-devices-dummy-test.device/start timed out.
autoStart # [  312.952269] systemd[1]: Timed out waiting for device /sys/subsystem/net/devices/dummy/test.
autoStart # [  312.958761] systemd[1]: sys-subsystem-net-devices-dummy-test.device: Job sys-subsystem-net-devices-dummy-test.device/start failed with result 'timeout'.
autoStart # [  312.975636] systemd[1]: Starting Container 'autoStart'...

This pr will fix this.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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