KVM: X86: Avoid traversing all the cpus for pv tlb flush when steal time is disabled

Avoid traversing all the cpus for pv tlb flush when steal time
is disabled since pv tlb flush depends on the field in steal time
for shared data.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim KrÄmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>