kvm: debugfs: add EXIT_REASON_PREEMPTION_TIMER to vcpu_stat

virt inclusion
category: feature
bugzilla: 46853
CVE: NA

Export EXIT_REASON_PREEMPTION_TIMER kvm exits to vcpu_stat debugfs.
Add a new column to vcpu_stat, and provide preemption_timer status to
virtualization detection tools.

Signed-off-by: chenjiajun <chenjiajun8@huawei.com>
Reviewed-by: Xiangyou Xie <xiexiangyou@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>