qemu: command: rework adding of default cpu model
    
    Certain XML features that aren't in the <cpu> block map to -cpu
    flags on the qemu cli. If one of these is specified but the user
    didn't explicitly pass an XML <cpu> model, we need to format a
    default model on the command line.
    
    The current code handles this by sprinkling this default cpu handling
    among all the different flag string formatting. Instead, switch it
    to do this just once.
    
    This alters some test output slightly: the previous code would
    write the default -cpu in some cases when no flags were actually
    added, so the output was redundant.
    
    Reviewed-by: Ján Tomko <jtomko@redhat.com>
    Signed-off-by: Cole Robinson <crobinso@redhat.com>