hw/arm/xlnx-versal-virt: split into base/concrete classes

hw/arm/xlnx-versal-virt: split into base/concrete classes

Split the xlnx-versal-virt machine type into a base abstract type and a
concrete type. There is no functional change. This is in preparation for
the versal2 machine.

Signed-off-by: Luc Michel <luc.michel@amd.com>
Reviewed-by: Francisco Iglesias <francisco.iglesias@amd.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20250926070806.292065-43-luc.michel@amd.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>