hw/riscv/virt.c: do not use RISCV_FEATURE_MMU in create_fdt_socket_cpus()

hw/riscv/virt.c: do not use RISCV_FEATURE_MMU in create_fdt_socket_cpus()

Read cpu_ptr->cfg.mmu directly. As a bonus, use cpu_ptr in
riscv_isa_string().

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20230222185205.355361-9-dbarboza@ventanamicro.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>