Description of problem:
Make sure there is a guest on host at least, Configure virt-who run at VDSM mode, then run virt-who, it will show "TypeError: 'str' object is not callable"

Version-Release number of selected component (if applicable):
subscription-manager-1.12.14-2.el6.x86_64
python-rhsm-1.12.5-2.el6.x86_64
virt-who-0.10-7.el6.noarch
katello-headpin-1.4.3.26-1.el6sam_splice.noarch
candlepin-0.9.6.5-1.el6sam.noarch

How reproducible:
Always

Steps to Reproduce:
Precondition: Make sure there is a guest on host at least

1. Register system to SAM server.
2. Configure virt-who run at VDSM mode 
VIRTWHO_DEBUG=1
VIRTWHO_BACKGROUND=1
VIRTWHO_INTERVAL=10
VIRTWHO_VDSM=1
3. Restart vdsmd service, then check the vdsmd service run normally.
[root@hp-z220-08 ~]# service vdsmd restart
[root@hp-z220-08 ~]# service vdsmd status
VDS daemon server is running
4. Restart virt-who service
5. Check the virt-who log at /var/log/rhsm/rhsm.log

Actual results:
2014-09-09 13:35:17,918 [ERROR]  @virtwho.py:136 - Unable to recover, retry in 5 seconds.
2014-09-09 13:35:23,015 [ERROR]  @virtwho.py:133 - Error in communication with subscription manager, trying to recover:
Traceback (most recent call last):
  File "/usr/share/virt-who/virtwho.py", line 125, in _send
    self._sendGuests(config, virtualGuests)
  File "/usr/share/virt-who/virtwho.py", line 152, in _sendGuests
    manager.sendVirtGuests(virtualGuests)
  File "/usr/share/virt-who/manager/subscriptionmanager/subscriptionmanager.py", line 104, in sendVirtGuests
    domains.sort(key=key)
TypeError: 'str' object is not callable

Expected results:
It shouldn't show up any error messages.Virt-who should run normally under the VDSM mode.

Additional info:
When there isn't guest on host, it run normally.
2014-09-09 13:33:20,387 [INFO]  @virtwho.py:442 - Using virt-who configuration: virt-who
2014-09-09 13:33:20,387 [DEBUG]  @virtwho.py:170 - Starting infinite loop with 10 seconds interval
2014-09-09 13:33:20,735 [INFO]  @subscriptionmanager.py:109 - Sending list of uuids: []