qemu: error out on missing machine type in configs
    
    Commit f1a89a8 allowed parsing configs from /etc/libvirt
    without validating the emulator capabilities.
    
    Check for the presence of a machine type in the qemu driver's
    post parse function instead of crashing.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1267256