The code added to validate CA certificates did not take into
account the possibility that the cacert.pem file can contain
multiple (concatenated) cert data blocks. Extend the code for
loading CA certs to use the gnutls APIs for loading cert lists.
Add test cases to check that multi-level trees of certs will
validate correctly.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>