Implement get_hypervisor_hostname for libvirt.

Implements get_hypervisor_hostname so that the
OS-EXT-SRV-ATTR:hypervisor_hostname extension can properly
display this information when using libvirt.

Fixes LP Bug #999729.

Change-Id: I124d48ca352dc225f88999046f9d212ffb9e4c6e

nova/tests/fakelibvirt.py

@@ -507,6 +507,9 @@ class Connection(object):
     def getVersion(self):
         return 14000
 
+    def getHostname(self):
+        return 'compute1'
+
     def getCapabilities(self):
         return '''<capabilities>
   <host>