crypto: allow default TLS priority to be chosen at build time

Modern gnutls can use a global config file to control the
crypto priority settings for TLS connections. For example
the priority string "@System" instructs gnutls to find the
priority setting named "SYSTEM" in the global config file.

Latest gnutls GIT codebase gained the ability to reference
multiple priority strings in the config file, with the first
one that is found to existing winning. This means it is now
possible to configure QEMU out of the box with a default
priority of "@qemu,SYSTEM", which says to look for the
settings "QEMU" first, and if not found, use the "SYSTEM"
settings.

To make use of this facility, we introduce the ability to
set the QEMU default priority at build time via a new
configure argument.  It is anticipated that distro vendors
will set this when building QEMU to a suitable value for
use with distro crypto policy setup. eg current Fedora
would run

 ./configure --tls-priority=@System

while future Fedora would run

 ./configure --tls-priority=@qemu,SYSTEM

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

configure

@@ -305,6 +305,7 @@ archipelago="no"
 gtk=""
 gtkabi=""
 gtk_gl="no"
+tls_priority="NORMAL"
 gnutls=""
 gnutls_rnd=""
 nettle=""
@@ -1096,6 +1097,8 @@ for opt do
   ;;
   --enable-gtk) gtk="yes"
   ;;
+  --tls-priority=*) tls_priority="$optarg"
+  ;;
   --disable-gnutls) gnutls="no"
   ;;
   --enable-gnutls) gnutls="yes"
@@ -1307,6 +1310,7 @@ Advanced options (experts only):
   --disable-blobs          disable installing provided firmware blobs
   --with-vss-sdk=SDK-path  enable Windows VSS support in QEMU Guest Agent
   --with-win-sdk=SDK-path  path to Windows Platform SDK (to build VSS .tlb)
+  --tls-priority           default TLS protocol/cipher priority string
 
 Optional features, enabled with --enable-FEATURE and
 disabled with --disable-FEATURE, default is enabled if available:
@@ -4802,6 +4806,7 @@ echo "SDL support       $sdl $(echo_version $sdl $sdlversion)"
 echo "GTK support       $gtk $(echo_version $gtk $gtk_version)"
 echo "GTK GL support    $gtk_gl"
 echo "VTE support       $vte $(echo_version $vte $vteversion)"
+echo "TLS priority      $tls_priority"
 echo "GNUTLS support    $gnutls"
 echo "GNUTLS rnd        $gnutls_rnd"
 echo "libgcrypt         $gcrypt"
@@ -5165,6 +5170,7 @@ if test "$gtk" = "yes" ; then
     echo "CONFIG_GTK_GL=y" >> $config_host_mak
   fi
 fi
+echo "CONFIG_TLS_PRIORITY=\"$tls_priority\"" >> $config_host_mak
 if test "$gnutls" = "yes" ; then
   echo "CONFIG_GNUTLS=y" >> $config_host_mak
 fi

crypto/tlssession.c

@@ -137,7 +137,7 @@ qcrypto_tls_session_new(QCryptoTLSCreds *creds,
         if (creds->priority != NULL) {
             prio = g_strdup_printf("%s:+ANON-DH", creds->priority);
         } else {
-            prio = g_strdup("NORMAL:+ANON-DH");
+            prio = g_strdup(CONFIG_TLS_PRIORITY ":+ANON-DH");
         }
 
         ret = gnutls_priority_set_direct(session->handle, prio, NULL);
@@ -167,7 +167,7 @@ qcrypto_tls_session_new(QCryptoTLSCreds *creds,
         QCryptoTLSCredsX509 *tcreds = QCRYPTO_TLS_CREDS_X509(creds);
         const char *prio = creds->priority;
         if (!prio) {
-            prio = "NORMAL";
+            prio = CONFIG_TLS_PRIORITY;
         }
 
         ret = gnutls_priority_set_direct(session->handle, prio, NULL);