KVM: vmx, pmu: accept 0 for host-initiated write to MSR_IA32_DS_AREA

Whenever an MSR is part of KVM_GET_MSR_INDEX_LIST, as is the case
for MSR_IA32_DS_AREA, it has to be always settable with KVM_SET_MSR.
Accept a zero value for these MSRs to obey the contract.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>