target/riscv: assert argument to set_satp_mode_max_supported is valid

target/riscv: assert argument to set_satp_mode_max_supported is valid

Check that the argument to set_satp_mode_max_supported is valid for
the MXL value of the CPU.  It would be a bug in the CPU definition
if it weren't.

In fact, there is such a bug in riscv_bare_cpu_init(): not just
SV64 is not a valid VM mode for 32-bit CPUs, SV64 is not a
valid VM mode at all, not yet at least.

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