Move the work of adding and removing TDP MMU pages to/from  "secondary"
data structures to helper functions. These functions will be built on in
future commits to enable MMU operations to proceed (mostly) in parallel.

No functional change expected.
Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210202185734.1680553-20-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>