KVM: selftests: Collect *all* dirty entries in each dirty_log_test iteration

KVM: selftests: Collect *all* dirty entries in each dirty_log_test iteration

Collect all dirty entries during each iteration of dirty_log_test by
doing a final collection after the vCPU has been stopped.  To deal with
KVM's destructive approach to getting the dirty bitmaps, use a second
bitmap for the post-stop collection.

Collecting all entries that were dirtied during an iteration simplifies
the verification logic *and* improves test coverage.

  - If a page is written during iteration X, but not seen as dirty until
    X+1, the test can get a false pass if the page is also written during
    X+1.

  - If a dirty page used a stale value from a previous iteration, the test
    would grant a false pass.

  - If a missed dirty log occurs in the last iteration, the test would fail
    to detect the issue.

E.g. modifying mark_page_dirty_in_slot() to dirty an unwritten gfn:

	if (memslot && kvm_slot_dirty_track_enabled(memslot)) {
		unsigned long rel_gfn = gfn - memslot->base_gfn;
		u32 slot = (memslot->as_id << 16) | memslot->id;

		if (!vcpu->extra_dirty &&
		    gfn_to_memslot(kvm, gfn + 1) == memslot) {
			vcpu->extra_dirty = true;
			mark_page_dirty_in_slot(kvm, memslot, gfn + 1);
		}
		if (kvm->dirty_ring_size && vcpu)
			kvm_dirty_ring_push(vcpu, slot, rel_gfn);
		else if (memslot->dirty_bitmap)
			set_bit_le(rel_gfn, memslot->dirty_bitmap);
	}

isn't detected with the current approach, even with an interval of 1ms
(when running nested in a VM; bare metal would be even *less* likely to
detect the bug due to the vCPU being able to dirty more memory).  Whereas
collecting all dirty entries consistently detects failures with an
interval of 700ms or more (the longer interval means a higher probability
of an actual write to the prematurely-dirtied page).

Link: https://lore.kernel.org/r/20250111003004.1235645-15-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>