Don't open the control socket until after the server socket is bound

This makes it easier for the tests (and supervisor) to guarantee to be
able to connect to the server socket.

Also this patch moves freeing the mirror supervisor into the server
thread.

tests/acceptance/flexnbd.rb

@@ -97,7 +97,9 @@ class ValgrindKillingExecutor
       when "line"
         @error.add_line( @text ) if @found
       when "error", "stack"
-        @killer.call( @error )
+        if @found
+          @killer.call( @error )
+        end
       when "pid"
         @error.pid=@text
       end
@@ -326,8 +328,10 @@ module FlexNBD
     def serve( file, *acl)
       cmd = serve_cmd( file, acl )
       run_serve_cmd( cmd )
+      sleep( 0.2 ) until File.exists?( ctrl )
     end
 
+
     def listen(file, *acl)
       run_serve_cmd( listen_cmd( file, acl ) )
     end