hw/nvram/fw_cfg: Factor fw_cfg_init_mem_internal() out

hw/nvram/fw_cfg: Factor fw_cfg_init_mem_internal() out

Factor fw_cfg_init_mem_internal() out of fw_cfg_init_mem_wide().
In fw_cfg_init_mem_wide(), assert DMA arguments are provided.
Callers without DMA have to use the fw_cfg_init_mem() helper.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20260108033051.777361-11-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>