Description of problem:
Before virt-who send mapping info to stage candlepin, stop virt-who service will result in "AttributeError: 'NoneType' object has no attribute'propertyCollector'"in rhsm.log

Version-Release number of selected component (if applicable):
virt-who-0.17-10.el7.noarch
subscription-manager-1.17.11-1.el7.x86_64
python-rhsm-1.17.7-1.el7.x86_64

How reproducible:
Always

Steps to Reproduce:
1. Register system to stage candlepin.
2. Configure virt-who run at esx mode.
[root@hp-z220-06 virt-who.d]# cat /etc/virt-who.d/esx 
[esx]
type=esx
server=10.73.2.95
username=administrator
password=Welcome1!
owner=7985933
env=7985933
3. Restart virt-who service, before virt-who send mapping info to stage candlepin, stop virt-who service
[root@hp-z220-06 virt-who.d]# service virt-who restart
Redirecting to /bin/systemctl restart  virt-who.service
[root@hp-z220-06 virt-who.d]# service virt-who stop
Redirecting to /bin/systemctl stop  virt-who.service

4. Check virt-who's log
[root@hp-z220-06 virt-who.d]# tail -f /var/log/rhsm/rhsm.log
2016-09-07 02:27:41,814 [virtwho.esx DEBUG] Esx-1(21082):MainThread @virt.py:run:364 - Virt backend 'esx' started
2016-09-07 02:27:41,815 [virtwho.esx DEBUG] Esx-1(21082):MainThread @esx.py:_prepare:128 - Log into ESX
2016-09-07 02:27:42,038 [virtwho.esx DEBUG] Esx-1(21082):MainThread @esx.py:_prepare:131 - Creating ESX event filter
2016-09-07 02:27:42,192 [virtwho.esx DEBUG] Esx-1(21082):MainThread @virt.py:enqueue:357 - Report for config "esx" gathered, putting to queue for sending
2016-09-07 02:27:42,205 [virtwho.main DEBUG] MainProcess(21075):MainThread @subscriptionmanager.py:_connect:124 - Authenticating with certificate: /etc/pki/consumer/cert.pem
2016-09-07 02:27:43,402 [virtwho.main DEBUG] MainProcess(21075):MainThread @subscriptionmanager.py:hypervisorCheckIn:172 - Checking if server has capability 'hypervisor_async'
2016-09-07 02:27:43,936 [virtwho.esx ERROR] Esx-1(21082):MainThread @virt.py:run:379 - Virt backend 'esx' fails with exception:
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/virtwho/virt/virt.py", line 372, in run
    self._run()
  File "/usr/lib/python2.7/site-packages/virtwho/virt/esx/esx.py", line 173, in _run
    _this=self.sc.propertyCollector,
AttributeError: 'NoneType' object has no attribute 'propertyCollector'
2016-09-07 02:27:43,936 [virtwho.esx INFO] Esx-1(21082):MainThread @virt.py:run:392 - Waiting 60 seconds before retrying backend 'esx'
2016-09-07 02:27:44,325 [virtwho.main DEBUG] MainProcess(21075):MainThread @executor.py:terminate:303 - virt-who is shutting down
2016-09-07 02:27:45,781 [virtwho.main DEBUG] MainProcess(21075):MainThread @subscriptionmanager.py:hypervisorCheckIn:184 - Server does not have 'hypervisors_async' capability
2016-09-07 02:27:45,781 [virtwho.main INFO] MainProcess(21075):MainThread @subscriptionmanager.py:hypervisorCheckIn:195 - Sending update in hosts-to-guests mapping for config "esx": 3 hypervisors and 2 guests found
2016-09-07 02:27:45,781 [virtwho.main DEBUG] MainProcess(21075):MainThread @subscriptionmanager.py:hypervisorCheckIn:196 - Host-to-guest mapping: {
    "7a384d56-085c-0e32-c7ad-f7181c3f6ddf": [], 
    "3e504d56-3982-037b-f3e6-669289892fa1": [], 
    "60554d56-20cb-f25e-7ab9-33e7e330dc15": [
        {
            "guestId": "42067ff0-8098-f4a4-8c53-826adb0e7b84", 
            "state": 5, 
            "attributes": {
                "active": 0, 
                "virtWhoType": "esx"
            }
        }, 
        {
            "guestId": "4206ee69-f83e-c97f-521f-4af794d5cde9", 
            "state": 1, 
            "attributes": {
                "active": 1, 
                "virtWhoType": "esx"
            }
        }
    ]
}
2016-09-07 02:27:46,813 [virtwho.main DEBUG] MainProcess(21075):MainThread @executor.py:send_report:101 - Report for config "esx" sent
2016-09-07 02:27:46,814 [virtwho.main DEBUG] MainProcess(21075):MainThread @__main__.py:main:19 - virt-who terminated

Actual results:
It will show "AttributeError" in the log.

Expected results:
It shouldn't show these error info in the log.

Additional info: