kvm: arm64: emulate the ID registers

hulk inclusion
category: feature
bugzilla: NA
CVE: NA

To emulate the ID registers, we need a place to storage the values of
the ID regsiters.  Maybe putting in kvm_arch_vcpu is a good idea.

This commit has no functional changes but only code refactor.  When
initializing a vcpu, get the values of the ID registers from
arm64_ftr_regs and storage them in kvm_arch_vcpu.  And we just read
the value from kvm_arch_vcpu when getting/setting the value of the ID
regs.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Peng Liang <liangpeng10@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>