All code now uses built-in UUID implementation. Remove the code of
libuuid and make --enable-uuid and --disable-uuid only print a message.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-Id: <1474432046-325-9-git-send-email-famz@redhat.com>