Description of problem:
When unregister system to satellite, it will show "Communication with server failed" as "RestlibException: undefined method `inject' for nil:NilClass" and "RemoteServerException: Server error attempting a POST to /rhsm/hypervisors?owner=ACME_Corporation&env=Library returned status 500"

Version-Release number of selected component (if applicable):
virt-who-0.14-3.el7.noarch
subscription-manager-1.15.9-2.el7.x86_64
python-rhsm-1.15.4-2.el7.x86_64
Satellite 6.1.0-20150731.0

How reproducible:
70%

Steps to Reproduce:
1. Register system to satellite
2. Configure virt-who run at esx mode and refresh interval is 5s, then restart virt-who service
3. Unregister system to satellite, then check the virt-who's log

Actual results:
There are two kinds of failed phenomenon when communicate with server. Sometime it will show "RestlibException", the log as the following
2015-08-11 14:33:29,801 [DEBUG] rhsmd:13892 @ga_loader.py:99 - ga_loader class GaImporterGtk3 loading virtual module subscription_manager.ga from None
2015-08-11 14:33:29,801 [DEBUG] rhsmd:13892 @ga_loader.py:99 - ga_loader class GaImporterGtk3 loading virtual module subscription_manager.ga.GObject from ['gi.repository', 'GObject']
2015-08-11 14:33:29,811 [DEBUG] rhsmd:13892 @ga_loader.py:99 - ga_loader class GaImporterGtk3 loading virtual module subscription_manager.ga.GLib from ['gi.repository', 'GLib']
2015-08-11 14:33:29,827 [ERROR]  @subscriptionmanager.py:171 - Communication with server failed:
Traceback (most recent call last):
  File "/usr/share/virt-who/manager/subscriptionmanager/subscriptionmanager.py", line 167, in hypervisorCheckIn
    result = self.connection.hypervisorCheckIn(config.owner, config.env, serialized_mapping)
  File "/usr/lib64/python2.7/site-packages/rhsm/connection.py", line 860, in hypervisorCheckIn
    res = self.conn.request_post(url, host_guest_mapping)
  File "/usr/lib64/python2.7/site-packages/rhsm/connection.py", line 612, in request_post
    return self._request("POST", method, params)
  File "/usr/lib64/python2.7/site-packages/rhsm/connection.py", line 537, in _request
    self.validateResponse(result, request_type, handler)
  File "/usr/lib64/python2.7/site-packages/rhsm/connection.py", line 579, in validateResponse
    raise RestlibException(response['status'], error_msg)
RestlibException: undefined method `inject' for nil:NilClass
2015-08-11 14:33:29,829 [ERROR]  @virtwho.py:170 - Unable to send data: Communication with subscription manager failed with code 500: undefined method `inject' for nil:NilClass
==============================================================
Sometimes it will show "RemoteServerException" , the failed log as the following:
2015-08-11 14:38:11,985 [ERROR]  @subscriptionmanager.py:171 - Communication with server failed:
Traceback (most recent call last):
  File "/usr/share/virt-who/manager/subscriptionmanager/subscriptionmanager.py", line 167, in hypervisorCheckIn
    result = self.connection.hypervisorCheckIn(config.owner, config.env, serialized_mapping)
  File "/usr/lib64/python2.7/site-packages/rhsm/connection.py", line 860, in hypervisorCheckIn
    res = self.conn.request_post(url, host_guest_mapping)
  File "/usr/lib64/python2.7/site-packages/rhsm/connection.py", line 612, in request_post
    return self._request("POST", method, params)
  File "/usr/lib64/python2.7/site-packages/rhsm/connection.py", line 537, in _request
    self.validateResponse(result, request_type, handler)
  File "/usr/lib64/python2.7/site-packages/rhsm/connection.py", line 585, in validateResponse
    handler=handler)
RemoteServerException: Server error attempting a POST to /rhsm/hypervisors?owner=ACME_Corporation&env=Library returned status 500

Expected results:
It shouldn't show these error in the rhsm.log.virt-who should communicate with satellite normally.

Additional info: