Added the flexnbd break command to stop mirroring

tests/acceptance/fakes/dest/break_after_hello.rb

@@ -0,0 +1,35 @@
+#!/usr/bin/env ruby
+# encoding: utf-8
+
+# Open a server, accept a client, then cancel the migration by issuing
+# a break command.
+
+require 'flexnbd/fake_dest'
+include FlexNBD
+
+addr, port, src_pid, _, _, sock = *ARGV
+server = FakeDest.new( addr, port )
+client = server.accept
+
+ctrl = UNIXSocket.open( sock )
+
+Process.kill("STOP", src_pid.to_i)
+ctrl.write( "break\n" )
+ctrl.close_write
+client.write_hello
+Process.kill("CONT", src_pid.to_i)
+
+fail "Unexpected control response" unless
+  ctrl.read =~ /0: mirror stopped/
+
+client2 = nil
+begin
+  client2 = server.accept( "Expected timeout" )
+  fail "Unexpected reconnection"
+rescue Timeout::Error
+  # expected
+end
+client.close
+
+exit(0)
+