KVM: x86/mmu: Factor out handling of removed page tables

Factor out the code to handle a disconnected subtree of the TDP paging
structure from the code to handle the change to an individual SPTE.
Future commits will build on this to allow asynchronous page freeing.

No functional change intended.

Reviewed-by: Peter Feiner <pfeiner@google.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Ben Gardon <bgardon@google.com>

Message-Id: <20210202185734.1680553-6-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>