hw/core/null-machine: Do not initialize unused chardev backends

hw/core/null-machine: Do not initialize unused chardev backends

The MachineClass uses an inverted logic (inherited from the
PC machines [*]) to create the chardev backends for the default
devices (see commits 998bbd74b9d..aa40fc9c964 and ac33f8fad14).

As the none-machine doesn't have any hardware device, it is
pointless to initialize chardev backends. Fix by setting the
'no_defaults' bits in its MachineClass.

Suggested-by: Thomas Huth <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Message-Id: <20200624105611[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>