Make QEMU sanity check a build blocker
    
    If it ever breaks, it can be skipped by setting the %qemu_sanity_check
    variable to 0.
    
    This ensures it does not bit-rot again in future and actually adds some
    value to the build.
    
    It has to be skipped for ARM for now due to inability to select the
    machine type.
    
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>