Auto-detect qemu-img qed support for unit tests

This will let downstream distros with qemu-img versions
that don't support the qed format pass unit tests without
having to patch this test out.

Change-Id: I50907c90686366cab12be072b53c65b048b7c510
(cherry picked from commit 93ae58c)

Author: eharney

glance/tests/unit/common/test_format_inspector.py

@@ -179,7 +179,14 @@ def test_from_file_reads_minimum(self):
         # a local file.
         self.assertLess(fmt.actual_size, file_size)
 
+    def qed_supported(self):
+        output = subprocess.check_output('qemu-img create --help', shell=True)
+        return b' qed ' in output
+
     def test_qed_always_unsafe(self):
+        if not self.qed_supported():
+            raise self.skipException('qed format not supported by qemu-img')
+
         img = self._create_img('qed', 10 * units.Mi)
         fmt = format_inspector.get_inspector('qed').from_file(img)
         self.assertTrue(fmt.format_match)