libvirt: Set "Disabled Reason" to None when enable nova compute

When use "nova host-update" to enable nova compute, the column of
"Disabled Reason" will be set as empty, this will make the
"Disabled Reason" is not consistent with other services.

+------------------+------+----------+---------+-----------------+
| Binary           | Host |   Zone   |   ...   | Disabled Reason |
+------------------+------+----------+---------+-----------------+
| nova-conductor   | jay1 | internal |   ...   | None            |
| nova-cert        | jay1 | internal |   ...   | None            |
| nova-scheduler   | jay1 | internal |   ...   | None            |
| nova-compute     | jay1 | nova     |   ...   |                 |
| nova-consoleauth | jay1 | internal |   ...   | None            |
+------------------+------+----------+---------+-----------------+

The fix was set the "Disabled Reason" to None when enable a nova
compute.

Change-Id: Ief1f21ba450812a240d30104624bcec92e3f989e
Partial-Bug: #1251803
Merge "libvirt: Set "Disabled Reason" to None when enable nova compute"