util: fix handling of unspecified port in URI

When no server name is provided in the URI, modern versions of libxml2
will set the port to '-1'. This is a change from behaviour with earlier
versions which set it to 0.

Libvirt expects the port to be 0 in these cases and as a result we get a
bug when connecting to URIs which lack a server name:

$ virsh  -c test+ssh:///default list
error: failed to connect to the hypervisor
error: Cannot recv data: Bad port '-1': Connection reset by peer

This libxml2 change was attempting to fix another bug identified by
libvirt where it didn't roundtrip URIs correctly in:

  https://github.com/GNOME/libxml2/commit/beb7281055dbf0ed4d041022a67c6c5cfd126f25

Essentially libxml2 was not expecting apps to look at the URI port
field when the server name is not provided. This was a reasonable
assumption, but none the less libvirt did look at it :-)

The fix is to ensure we explicitly set port to 0 when server name
is not present, avoiding undefined behaviour for the port field in
libxml2.
Reviewed-by: NErik Skultety <eskultet@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

src/util/viruri.c

@@ -171,7 +171,16 @@ virURIParse(const char *uri)
         goto error;
     if (VIR_STRDUP(ret->server, xmluri->server) < 0)
         goto error;
-    ret->port = xmluri->port;
+    /* xmluri->port value is not defined if server was
+     * not given. Modern versions libxml2 fill port
+     * differently to old versions in this case, so
+     * don't rely on it. eg libxml2 git commit:
+     *   beb7281055dbf0ed4d041022a67c6c5cfd126f25
+     */
+    if (!ret->server || STREQ(ret->server, ""))
+        ret->port = 0;
+    else
+        ret->port = xmluri->port;
     if (VIR_STRDUP(ret->path, xmluri->path) < 0)
         goto error;
 #ifdef HAVE_XMLURI_QUERY_RAW