openvswitch: better integration with systemd

Systemd dependencies for scripted mode
were refactored according to analysis in #34586.

networking.vswitches can now be used with systemd-networkd,
although they are not supported by the daemon, a nixos receipe
creates the switch and attached required interfaces (just like
the scripted version).

Vlans and internal interfaces are implemented following the
  template format i.e. each interface is
described using an attributeSet (vlan and type at the moment).
If vlan is present, then interface is added to the vswitch with
given tag (access mode). Type internal enabled vswitch to create
interfaces (see openvswitch docs).

Added configuration for configuring supported openFlow version on
the vswitch

This commit is a split from the original PR #35127.

nixos/modules/virtualisation/libvirtd.nix

@@ -219,7 +219,7 @@ in {
       wantedBy = [ "multi-user.target" ];
       requires = [ "libvirtd-config.service" ];
       after = [ "systemd-udev-settle.service" "libvirtd-config.service" ]
-              ++ optional vswitch.enable "vswitchd.service";
+              ++ optional vswitch.enable "ovs-vswitchd.service";
 
       environment.LIBVIRTD_ARGS = ''--config "${configFile}" ${concatStringsSep " " cfg.extraOptions}'';