go.tools/cmd/vet: Update canonical method check for new xml.Marshaler/Unmarshaler interfaces
go.tools/cmd/vet: Update canonical method check for new xml.Marshaler/Unmarshaler interfaces

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/14484043

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

webfd/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,skyportsystems/go.tools,8limbs/gotools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,8limbs/gotools,bryanxu/go-zh.tools,bryanxu/go-zh.tools