target/riscv: rvv-1.0: check MSTATUS_VS when accessing vector csr registers

target/riscv: rvv-1.0: check MSTATUS_VS when accessing vector csr registers

If VS field is off, accessing vector csr registers should raise an
illegal-instruction exception.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-12-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>