x86/kvm: Don't bother __pv_cpu_mask when !CONFIG_SMP

Enable PV TLB shootdown when !CONFIG_SMP doesn't make sense. Let's
move it inside CONFIG_SMP. In addition, we can avoid define and
alloc __pv_cpu_mask when !CONFIG_SMP and get rid of 'alloc' variable
in kvm_alloc_cpumask.

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Message-Id: <1617941911-5338-1-git-send-email-wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>