Revert commit 55e6d8cd.

This fix for https://bugzilla.redhat.com/show_bug.cgi?id=1267256
unconditionally required a machine type for all machine types
even though qemu is the only emulator using them.

Revert it to fix persistent configs for drivers with no machine type:
https://www.redhat.com/archives/libvir-list/2016-February/msg01228.html