Fix ELB instance health response parsing for multiple instances case

There seems to be an issue with XML parsing of ELB instance health
responses when there are multiple instances described in the
response. In particular, we currently seem to get back only 1 instance
state with the current code, where we expect all of them. This PR
attempts to fix the issue.

For an isolated example of the believed XML issue, see: http://play.golang.org/p/M7qb5paODi.

I suspect there are similar issues in the elb package, but want to see
where this initial PR goes first...