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>