brcompatd: Don't remove nonexistent ports if vswitchd will create them.

brcompatd: Don't remove nonexistent ports if vswitchd will create them.

There is a race between brcompatd and vswitchd for internal ports, e.g.
in this scenario:

	1. cfg-mod adds "bridge.xenbr0.port=xenbr0".
	2. vswitchd creates xenbr0.

we can have brcompatd slip in between them:

	2.5. brcompatd notices that there is no network device xenbr0
             and deletes the new line from the config file.

For the local port and other internal ports we don't want brcompatd
interfering, so this commit makes it ignore them.

Bug #1314.