x86/kvm: Unify kvm_pv_guest_cpu_reboot() with kvm_guest_cpu_offline()

Simplify the code by making PV features shutdown happen in one place.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210414123544.1060604-6-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>