Move supermin appliance to a subpackage
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1989514
    
    Move the appliance to a subpackage (libguestfs-appliance).  The main
    package depends on this new subpackage so in effect nothing changes
    for most users.
    
    However this allows the appliance to be replaced if there exists a
    package called "libguestfs-noappliance".  This package is not provided
    anywhere, you have to provide the dependency or make the package
    yourself.  But if you do this then libguestfs won't install the
    appliance and you are free to replace it with (eg) a fixed appliance.