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.

Or perhaps I'm just missing something, sorry for the noise in that case. @tomprince, do you mind take a look? (Should this be rebased to the 0.8.8 branch?)


(This was discovered thanks to @elmirjagudin's work on enabling pylint.)