target/riscv: generate virtual instruction exception

target/riscv: generate virtual instruction exception

This patch adds a mechanism to generate a virtual instruction
instruction exception instead of an illegal instruction exception
during instruction decode when virt is enabled.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221016124726.102129-4-mchitale@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>