libvirt: Fix log message when disable/enable a host

When disable/enable a host, the log message is not correct. As
nova compute will log message saying the host was disabled when
enable a host and log message saying the host was enabled when
disable a host.

We should do as this:
update to disable -> log message should be the host was disabled
update to enable -> log message should be the host was enabled

Change-Id: I394e92fe6fe5ffa06464f1ba93e9fc2acec942ef
Closes-Bug: #1251822
Merge "libvirt: Fix log message when disable/enable a host"