Don't ignore host state changes when the change is to the state "unknown". This

happens when a host was scanned in the A scan but wasn't scanned in the B scan.
I previously had it ignore such changes using the logic that the diff should be
like scan aggregation: no new information means no state change. But I think
it's more useful to see those changes in which hosts were scanned.

This is analogous to r10263, which did the same thing for port state changes.

ndiff/README

@@ -36,6 +36,12 @@ Here is a sample of the text output:
 		Add ipv4 address 10.242.160.155.
 		Add hostname ywnleu-108.example.com.
 		1000 tcp ports are filtered.
+	fiyrownc-307.example.com (10.65.53.252):
+		Host is unknown, was up.
+		Remove ipv4 address 10.65.53.252.
+		Remove hostname fiyrownc-307.example.com.
+		8089/tcp is unknown, was open.
+		999 tcp ports changed state from filtered to unknown.
 
 Here is an abbreviated sample of the XML output:
 

ndiff/ndiff

@@ -447,7 +447,6 @@ def scan_diff(a, b):
     for id in all_host_ids:
         host_a = a_hosts.get(id)
         host_b = b_hosts.get(id)
-        if host_b is not None and host_b.state != Host.UNKNOWN:
         h_diff = host_diff(host_a or Host(), host_b or Host())
         if len(h_diff) > 0:
             diff.append((host_a or host_b, h_diff))

ndiff/ndifftest.py

@@ -487,7 +487,7 @@ class scan_diff_test(unittest.TestCase):
         for host, h_diff in diff:
             for hunk in h_diff:
                 if hunk.type == HOST_STATE_CHANGE:
-                    self.assertTrue(hunk.a_state == Port.UNKNOWN)
+                    self.assertTrue(hunk.a_state == Host.UNKNOWN)
                     self.assertTrue(hunk.b_state == u"up")
                     break
             else:
@@ -499,7 +499,14 @@ class scan_diff_test(unittest.TestCase):
         b = Scan()
         b.load_from_file("test-scans/empty.xml")
         diff = scan_diff(a, b)
-        self.assertTrue(len(diff) == 0)
+        for host, h_diff in diff:
+            for hunk in h_diff:
+                if hunk.type == HOST_STATE_CHANGE:
+                    self.assertTrue(hunk.a_state == u"up")
+                    self.assertTrue(hunk.b_state == Port.UNKNOWN)
+                    break
+            else:
+                fail("No host state change found.")
 
     def test_diff_is_effective(self):
         """Test that a scan diff is effective.