Current implementation keeps userspace input of CPUID configuration and
cpuid->nent even if kvm_update_cpuid() fails. Reset vcpu->arch.cpuid_nent
to 0 for the case of failure as a simple fix.

Besides, update the doc to explicitly state that if IOCTL SET_CPUID*
fail KVM gives no gurantee that previous valid CPUID configuration is
kept.
Signed-off-by: NXiaoyao Li <xiaoyao.li@intel.com>
Message-Id: <20200708065054.19713-2-xiaoyao.li@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>