Handle a failed disconnect correctly

If the sender disconnects its socket before sending the disconnect
message, the destination should restart the migration process.  This
patch makes sure that happens.

tests/acceptance/flexnbd/fake_dest.rb

@@ -94,7 +94,7 @@ module FlexNBD
       end
 
 
-      def receive_mirror
+      def receive_mirror( opts = {} )
         write_hello()
         loop do
           req = read_request
@@ -103,7 +103,7 @@ module FlexNBD
             read_data( req[:len] )
             write_reply( req[:handle] )
           when 65536
-            write_reply( req[:handle] )
+            write_reply( req[:handle], opts[:err] == :entrust ? 1 : 0 )
             break
           else
             raise "Unexpected request: #{req.inspect}"

tests/acceptance/flexnbd/fake_source.rb

@@ -50,12 +50,27 @@ module FlexNBD
       send_request( 65536, handle )
     end
 
+    def write_disconnect_request( handle="myhandle" )
+      send_request( 2, handle )
+    end
+
 
     def write_data( data )
       @sock.write( data )
     end
 
 
+    def send_mirror
+      read_hello()
+      write_write_request( 0, 8 )
+      write_data( "12345678" )
+      read_response()
+      write_entrust_request()
+      read_response()
+      write_disconnect_request()
+      close()
+    end
+
 
     def read_response
       magic = @sock.read(4)