Use a more specific error reporting invalid disk hardware

Instead of using NovaException for reporting invalid user
specified disk hardware, use UnsupportedHardware and
UnsupportedVirtType exceptions.

Change-Id: I68ed1c0569bd0c0679e9fff3ffd436197f78a9f0
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

nova/tests/test_libvirt_blockinfo.py

@@ -421,7 +421,7 @@ class LibvirtBlockInfoTest(test.TestCase):
         self.assertEqual(bus, 'usb')
 
         image_meta = {'properties': {'disk_bus': 'xen'}}
-        self.assertRaises(exception.NovaException,
+        self.assertRaises(exception.UnsupportedHardware,
                           blockinfo.get_disk_bus_for_device_type,
                           'kvm',
                           image_meta)