kvm: x86: implement KVM PM-notifier

Implement PM hibernation/suspend prepare notifiers so that KVM
can reliably set PVCLOCK_GUEST_STOPPED on VCPUs and properly
suspend VMs.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Message-Id: <20210606021045.14159-2-senozhatsky@chromium.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>