block/curl: only restrict protocols with libcurl>=7.19.4

The curl_easy_setopt(state->curl, CURLOPT_PROTOCOLS, ...) interface was
introduced in libcurl 7.19.4.  Therefore we cannot protect against
CVE-2013-0249 when linking against an older libcurl.

This fixes the build failure introduced by
fb6d1bbd246c7a57ef53d3847ef225cd1349d602.

Reported-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Andreas Färber <andreas.faeber@web.de>
Message-id: 1360743934-8337-1-git-send-email-stefanha@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
