Now that the fast invalidate mechanism has been reintroduced, restore
the performance tweaks for fast invalidation that existed prior to its
removal.

Paraphrashing the original changelog:

  Introduce a per-VM list to track obsolete shadow pages, i.e. pages
  which have been deleted from the mmu cache but haven't yet been freed.
  When page reclaiming is needed, zap/free the deleted pages first.

This reverts commit 52d5dedc.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>