When _set_host_enabled() in virt/libvirt/driver.py
is called to change service status of a host from
enabled to disabled without providing disable_reason, then
a TypeError occurs while concatenating disabled_reason with
DISABLE_PREFIX. This prevents the service status of the host
from being updated.
This patch handles the case when disable_reason is None.
Also, unit tests have been fixed to validate the code path for
changing the disabled state of a host.
Closes-Bug: #1623738
Change-Id: Ib131f73444234723ef8bbb15bb4bd4a62a83320d
0683fcd3f5