kvm: debugfs: Export vcpu stat via debugfs

euleros inclusion
category: feature
bugzilla: NA
CVE: NA

This patch create debugfs entry for vcpu stat.
The entry path is /sys/kernel/debug/kvm/vcpu_stat.
And vcpu_stat contains partial kvm exits items of vcpu, include:
	pid, hvc_exit_stat, wfe_exit_stat, wfi_exit_stat,
	mmio_exit_user, mmio_exit_kernel, exits

Currently, The maximum vcpu limit is 1024.

From this vcpu_stat, user can get the number of these kvm exits items
over a period of time, which is helpful to monitor the virtual machine.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: chenjiajun <chenjiajun8@huawei.com>
Reviewed-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>