fix qemu-img version dependent tests

while backporting Ia34203f246f0bc574e11476287dfb33fda7954fe

We observed that several of the tests showed distro specific
behavior depending on if qemu was installed in the test env,
what version is installed and how it was compiled

This change ensures that if qemu is present that it
supprot the required formats otherwise it skips the test.

Change-Id: I131996cdd7aaf1f52d4caac33b153753ff6db869
(cherry picked from commit cc2514d02e)

nova/tests/unit/image/test_format_inspector.py

@@ -111,18 +111,22 @@ class TestFormatInspectors(test.NoDBTestCase):
         if fmt == 'iso':
             return self._create_iso(size, subformat)
 
-        # these tests depend on qemu-img
-        # being installed and in the path,
-        # if it is not installed, skip
-        try:
-            subprocess.check_output('qemu-img --version', shell=True)
-        except Exception:
-            self.skipTest('qemu-img not installed')
-
         if fmt == 'vhd':
             # QEMU calls the vhd format vpc
             fmt = 'vpc'
 
+        # these tests depend on qemu-img being installed and in the path,
+        # if it is not installed, skip. we also need to ensure that the
+        # format is supported by qemu-img, this can vary depending on the
+        # distribution so we need to check if the format is supported via
+        # the help output.
+        try:
+            subprocess.check_output(
+                'qemu-img --help | grep %s' % fmt, shell=True)
+        except Exception:
+            self.skipTest(
+                'qemu-img not installed or does not support %s format' % fmt)
+
         if options is None:
             options = {}
         opt = ''