crypto: add support for TLS priority string override

The gnutls default priority is either "NORMAL" (most historical
versions of gnutls) which is a built-in label in gnutls code,
or "@SYSTEM" (latest gnutls on Fedora at least) which refers
to an admin customizable entry in a gnutls config file.

Regardless of which default is used by a distro, they are both
global defaults applying to all applications using gnutls. If
a single application on the system needs to use a weaker set
of crypto priorities, this potentially forces the weakness onto
all applications. Or conversely if a single application wants a
strong default than all others, it can't do this via the global
config file.

This adds an extra parameter to the tls credential object which
allows the mgmt app / user to explicitly provide a priority
string to QEMU when configuring TLS.

For example, to use the "NORMAL" priority, but disable SSL 3.0
one can now configure QEMU thus:

  $QEMU -object tls-creds-x509,id=tls0,dir=/home/berrange/qemutls,\
                priority="NORMAL:-VERS-SSL3.0" \
        ..other args...

If creating tls-creds-anon, whatever priority the user specifies
will always have "+ANON-DH" appended to it, since that's mandatory
to make the anonymous credentials work.

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

crypto/tlscreds.c

@@ -178,6 +178,27 @@ qcrypto_tls_creds_prop_get_dir(Object *obj,
 }
 
 
+static void
+qcrypto_tls_creds_prop_set_priority(Object *obj,
+                                    const char *value,
+                                    Error **errp G_GNUC_UNUSED)
+{
+    QCryptoTLSCreds *creds = QCRYPTO_TLS_CREDS(obj);
+
+    creds->priority = g_strdup(value);
+}
+
+
+static char *
+qcrypto_tls_creds_prop_get_priority(Object *obj,
+                                    Error **errp G_GNUC_UNUSED)
+{
+    QCryptoTLSCreds *creds = QCRYPTO_TLS_CREDS(obj);
+
+    return g_strdup(creds->priority);
+}
+
+
 static void
 qcrypto_tls_creds_prop_set_endpoint(Object *obj,
                                     int value,
@@ -216,6 +237,10 @@ qcrypto_tls_creds_class_init(ObjectClass *oc, void *data)
                                    qcrypto_tls_creds_prop_get_endpoint,
                                    qcrypto_tls_creds_prop_set_endpoint,
                                    NULL);
+    object_class_property_add_str(oc, "priority",
+                                  qcrypto_tls_creds_prop_get_priority,
+                                  qcrypto_tls_creds_prop_set_priority,
+                                  NULL);
 }
 
 
@@ -234,6 +259,7 @@ qcrypto_tls_creds_finalize(Object *obj)
     QCryptoTLSCreds *creds = QCRYPTO_TLS_CREDS(obj);
 
     g_free(creds->dir);
+    g_free(creds->priority);
 }