block: curl: Allow passing cookies via QCryptoSecret

Since cookies can contain sensitive data (session ID, etc ...) it is
desired to hide them from the prying eyes of users. Add a possibility to
pass them via the secret infrastructure.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1447413

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: f4a22cdebdd0bca6a13a43a2a6deead7f2ec4bb3.1493906281.git.pkrempa@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
