qemu: Explicitly check for gnutls_rnd()

Our use of gnutls_rnd(), introduced with commit ad7520e8, is
conditional to the availability of the <gnutls/crypto.h> header
file.

Such check, however, turns out not to be strict enough, as there
are some versions of GnuTLS (eg. 2.8.5 from CentOS 6) that provide
the header file, but not the function itself, which was introduced
only in GnuTLS 2.12.0.

Introduce an explicit check for the function.

configure.ac

@@ -1289,6 +1289,13 @@ if test "x$with_gnutls" != "xno"; then
     with_gnutls=yes
   fi
 
+  dnl GNUTLS_CFLAGS and GNUTLS_LIBS have probably been updated above,
+  dnl and we need the final values for function probing to work
+  CFLAGS="$old_CFLAGS $GNUTLS_CFLAGS"
+  LIBS="$old_LIBS $GNUTLS_LIBS"
+
+  AC_CHECK_FUNCS([gnutls_rnd])
+
   CFLAGS="$old_CFLAGS"
   LIBS="$old_LIBS"
 fi

src/qemu/qemu_domain.c

@@ -635,8 +635,8 @@ qemuDomainGenerateRandomKey(size_t nbytes)
     if (VIR_ALLOC_N(key, nbytes) < 0)
         return NULL;
 
-#if HAVE_GNUTLS_CRYPTO_H
-    /* Generate a master key using gnutls if possible */
+#if HAVE_GNUTLS_RND
+    /* Generate a master key using gnutls_rnd() if possible */
     if ((ret = gnutls_rnd(GNUTLS_RND_RANDOM, key, nbytes)) < 0) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
                        _("failed to generate master key, ret=%d"), ret);
@@ -644,7 +644,7 @@ qemuDomainGenerateRandomKey(size_t nbytes)
         return NULL;
     }
 #else
-    /* If we don't have gnutls, we will generate a less cryptographically
+    /* If we don't have gnutls_rnd(), we will generate a less cryptographically
      * strong master key from /dev/urandom.
      */
     if ((ret = virRandomBytes(key, nbytes)) < 0) {