KVM: x86/pmu: Don't overwrite the pmu->global_ctrl when refreshing

Assigning a value to pmu->global_ctrl just to set the value of
pmu->global_ctrl_mask is more readable but does not conform to the
specification. The value is reset to zero on Power up and Reset but
stays unchanged on INIT, like most other MSRs.

Signed-off-by: Like Xu <likexu@tencent.com>
Message-Id: <20220510044407.26445-1-likexu@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>