RISC-V: KVM: Fix privilege mode setting in kvm_riscv_vcpu_trap_redirect()

The kvm_riscv_vcpu_trap_redirect() should set guest privilege mode
to supervisor mode because guest traps/interrupts are always handled
in virtual supervisor mode.

Fixes: 9f701326 ("RISC-V: KVM: Handle MMIO exits for VCPU")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>