target/riscv: store RISCVCPUDef struct directly in the class

target/riscv: store RISCVCPUDef struct directly in the class

Prepare for adding more fields to RISCVCPUDef and reading them in
riscv_cpu_init: instead of storing the misa_mxl_max field in
RISCVCPUClass, ensure that there's always a valid RISCVCPUDef struct
and go through it.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>