KVM: x86: Fold guts of kvm_arch_sync_events() into kvm_arch_pre_destroy_vm()

Fold the guts of kvm_arch_sync_events() into kvm_arch_pre_destroy_vm(), as
the kvmclock and PIT background workers only need to be stopped before
destroying vCPUs (to avoid accessing vCPUs as they are being freed); it's
a-ok for them to be running while the VM is visible on the global vm_list.

Note, the PIT also needs to be stopped before IRQ routing is freed
(because KVM's IRQ routing is garbage and assumes there is always non-NULL
routing).

Opportunistically add comments to explain why KVM stops/frees certain
assets early.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250224235542.2562848-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>