libvirt.conf: Add test for host_uuid option

Test host_uuid parameter in libvird.conf. 
This PR depends on https://github.com/autotest/virt-test/pull/1769.

1) Change host_uuid in libvirtd.conf;
2) Restart libvirt daemon;
3) Check if libvirtd successfully started;
4) Check current host UUID by `virsh capabilities`;

Signed-off-by: Hao Liu <hliu@redhat.com>
@yangdongsheng , I used `libvirtd.restart()` when calling `config.sync()` or `config.restore()` and came to an obstacle when restarting libvirtd too frequently. 
Apparently systemd have a limit for starting services defined in `/etc/systemd/user.conf` as `DefaultStartLimitInterval=10s` and `DefaultStartLimitBurst=5`.
If I start service like `libvirtd` too quickly, It failed with:
```
# systemctl restart libvirtd
Job for libvirtd.service failed. See 'systemctl status libvirtd.service' and 'journalctl -xn' for details.

# systemctl status libvirtd.service
libvirtd.service - Virtualization daemon
   Loaded: loaded (/usr/lib/systemd/system/libvirtd.service; enabled)
   Active: failed (Result: start-limit) since Wed 2014-07-23 11:15:31 CST; 1min 7s ago
  Process: 1489 ExecStart=/usr/sbin/libvirtd $LIBVIRTD_ARGS (code=exited, status=0/SUCCESS)
 Main PID: 1489 (code=exited, status=0/SUCCESS)

Jul 23 11:15:30 celeron systemd[1]: Stopping Virtualization daemon...
Jul 23 11:15:30 celeron systemd[1]: Starting Virtualization daemon...
Jul 23 11:15:30 celeron systemd[1]: Started Virtualization daemon.
Jul 23 11:15:31 celeron libvirtd[1489]: libvirt version: 1.1.1, package: 29.el7 (Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>, 2014-03-24-06:10:00)
Jul 23 11:15:31 celeron systemd[1]: Stopping Virtualization daemon...
Jul 23 11:15:31 celeron systemd[1]: Starting Virtualization daemon...
Jul 23 11:15:31 celeron systemd[1]: libvirtd.service start request repeated too quickly, refusing to start.
Jul 23 11:15:31 celeron systemd[1]: Failed to start Virtualization daemon.
Jul 23 11:15:31 celeron systemd[1]: Unit libvirtd.service entered failed state.
```

I was looking for an solution by trying to inspect the result of a `systemctl status xxx` after failure of `systemctl start|restart xxx`. But it seems it's impossible to implemented to the current `service.Factory` mode. Is there any suggestions to make this appropriate?