RISCV: wrong check for vector extension ELEN value.
Description of problem:
When checking if the vector extension ELEN value is in the range [8, 64], the lower bound check uses the `vlen` field instead of the `elen` one: https://gitlab.com/qemu-project/qemu/-/blob/master/target/riscv/cpu.c#L885.
Additional information:
This is basically just a typo I found while reading the code, I do not have a real case scenario which resulted in this check to fail.