target/riscv: fix for virtual instr exception

target/riscv: fix for virtual instr exception

commit fb3f3730e4 added mechanism to generate virtual instruction
exception during instruction decode when virt is enabled.

However in some situations, illegal instruction exception can be raised
due to state of CPU. One such situation is implementing branch tracking.
[1] An indirect branch if doesn't land on a landing pad instruction, then
cpu must raise an illegal instruction exception.
Implementation would raise such expcetion due to missing landing pad inst
and not due to decode. Thus DisasContext must have `virt_inst_excp`
initialized to false during DisasContxt initialization for TB.

[1] - https://github.com/riscv/riscv-cfi

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230127191758.755844-1-debug@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>