Revert "Error out on missing machine type in machine configs"
    
    Revert commit 55e6d8cd9eac7eb2aaa4d221585e9402cf7269d5.
    
    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