block/curl: disable extra protocols to prevent CVE-2013-0249

There is a buffer overflow in libcurl POP3/SMTP/IMAP.  The workaround is
simple: disable extra protocols so that they cannot be exploited.  Full
details here:

  http://curl.haxx.se/docs/adv_20130206.html

QEMU only cares about HTTP, HTTPS, FTP, FTPS, and TFTP.  I have tested
that this fix prevents the exploit on my host with
libcurl-7.27.0-5.fc18.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
