RISC-V: KVM: Record number of signal exits as a vCPU stat

Record a statistic indicating the number of times a vCPU has exited
due to a pending signal.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>