util: set error if DAD is not finished
If DAD not finished in 5 seconds, user will get an
unknown error like this:
# virsh net-start ipv6
error: Failed to start network ipv6
error: An error occurred, but the cause is unknown
Call virReportError to set an error.
Signed-off-by: Luyao Huang <lhuang@redhat.com>