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

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

BugLink: https://bugs.launchpad.net/bugs/1920944
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>
(backported from commit 384fc672f528d3b84eacd9a86ecf35df3363b8ba)
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: Guilherme G. Piccoli <gpiccoli@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>