hw/m68k: convert 'virt' machine definitions to use new macros

hw/m68k: convert 'virt' machine definitions to use new macros

This changes the DEFINE_VIRT_MACHINE macro to use the common
helpers for constructing versioned symbol names and strings,
bringing greater consistency across targets.

A DEFINE_VIRT_MACHINE_AS_LATEST helper is added so that it
is not required to pass 'false' for every single historical
machine type.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240620165742.1711389-6-berrange@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>