Convert XML structure initializers to use keys.

Created by: timburks

The unkeyed initializing was causing compilation failures on Google App Engine. Quoting a golang-nuts forum post by David Symonds of the App Engine team:

"App Engine has been rejecting app code using unkeyed composite struct literals since early 2012. The issue is that App Engine wants to accept correct Go code at time T, but then still be able to recompile it at time T+n, even if the language or standard libraries have evolved in a backward-compatible way between those two points in time. For that reason, it is in general strongly recommended to use keyed composite struct literals in Go code that references types in other packages, and we enforce that recommendation on App Engine."