ec2buildslave: Fix import error with EC2LatentBuildSlave

The migration module buildbot.ec2buildslave tried to import
EC2LatentBuildSlave from buildbot.buildslave.libvirt, but that class
lives in buildbot.buildslave.ec2, causing a import error when loading
the module.
Merge branch 'topic/ec2buildslave_import_error' of git://github.com/olof/buildbot