Fix logging libvirt error on python 3

This patch fixes error caused by absence of "message"
attribute in libvirtError when running on python 3.
ceilometer-agent-compute service fails to fetch
domain metadata and tries to log libvirt error,
during logging the following exception is raised and
service fails:
AttributeError: 'libvirtError' object has no attribute
'message'

Change-Id: I56e74d1cb30310db104c850e74b2d422d3aeb966

ceilometer/compute/discovery.py

@@ -24,7 +24,6 @@ from oslo_config import cfg
 from oslo_log import log
 from oslo_utils import timeutils
 
-
 try:
     import libvirt
 except ImportError:
@@ -147,7 +146,7 @@ class InstanceDiscovery(plugin_base.DiscoveryBase):
                     raise
                 LOG.error(
                     "Fail to get domain uuid %s metadata, libvirtError: %s",
-                    domain.UUIDString(), e.message)
+                    domain.UUIDString(), e)
                 continue
 
             full_xml = etree.fromstring(domain.XMLDesc())