Autopkgtest: Trigger LXC from linux only on valid architectures

If the linux source package does not build any binaries on the given
architecture, don't trigger an LXC test for it.

autopkgtest.py

@@ -222,11 +222,16 @@ class AutoPackageTest(object):
         # Hardcode linux → lxc trigger until we get a more flexible
         # implementation: https://bugs.debian.org/779559
         if src.startswith('linux-meta') and 'lxc' not in reported_pkgs:
-            try:
-                tests.append(('lxc', self.britney.sources['testing']['lxc'][VERSION]))
-            except KeyError:
-                # no lxc package in that series? *shrug*, then not
-                pass
+            # does this have any image on this arch?
+            for b in srcinfo[BINARIES]:
+                p, a = b.split('/', 1)
+                if a == arch and '-image' in p:
+                    try:
+                        tests.append(('lxc', self.britney.sources['testing']['lxc'][VERSION]))
+                    except KeyError:
+                        # no lxc package in that series? *shrug*, then not
+                        pass
+                    break
 
         tests.sort(key=lambda s_v: s_v[0])
         return tests

tests/test_autopkgtest.py

@@ -1310,8 +1310,19 @@ fancy 1 i386 linux-meta-lts-grumpy 1
                       testsuite='autopkgtest')
 
         self.do_test(
-            [('linux-generic', {'Source': 'linux-meta'}, None)],
-            {'linux-meta': (False, {'lxc 1': {'amd64': 'RUNNING', 'i386': 'RUNNING'}})})
+            [('linux-image', {'Source': 'linux-meta'}, None),
+             ('linux-image-64only', {'Source': 'linux-meta-64only', 'Architecture': 'amd64'}, None),
+            ],
+            {'linux-meta': (False, {'lxc 1': {'amd64': 'RUNNING', 'i386': 'RUNNING'}}),
+             'linux-meta-64only': (False, {'lxc 1': {'amd64': 'RUNNING'}})
+            })
+
+        self.assertEqual(
+            self.amqp_requests,
+            set(['debci-series-i386:lxc {"triggers": ["linux-meta/1"]}',
+                 'debci-series-amd64:lxc {"triggers": ["linux-meta/1"]}',
+                 'debci-series-amd64:lxc {"triggers": ["linux-meta-64only/1"]}']))
+
 
     def test_gcc(self):
         '''gcc only triggers some key packages'''