libxl: make devid a type so it is initialized properly

Previously device ids in libxl were treated as integers meaning they
were being initialized to 0, which is a valid device id. This patch
makes devid its own type in libxl and initializes it to -1, an invalid
value.

This fixes a bug where if you try to do a xl DEV-attach multiple
time it will continuously try to reattach device 0 instead of
generating a new device id.

Signed-off-by: Matthew Fioravante <matthew.fioravante@jhuapl.edu>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

xen-unstable changeset: 25988:8a073eb1e849
Backport-requested-by: Ian Campbell <Ian.Campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/python/xen/lowlevel/xl/xl.c

@@ -281,6 +281,11 @@ int attrib__libxl_domid_set(PyObject *v, libxl_domid *domid) {
     return 0;
 }
 
+int attrib__libxl_devid_set(PyObject *v, libxl_devid *devid) {
+   *devid = PyInt_AsLong(v);
+   return 0;
+}
+
 int attrib__struct_in_addr_set(PyObject *v, struct in_addr *pptr)
 {
     PyErr_SetString(PyExc_NotImplementedError, "Setting in_addr");
@@ -342,6 +347,10 @@ PyObject *attrib__libxl_domid_get(libxl_domid *domid) {
     return PyInt_FromLong(*domid);
 }
 
+PyObject *attrib__libxl_devid_get(libxl_devid *devid) {
+    return PyInt_FromLong(*devid);
+}
+
 PyObject *attrib__struct_in_addr_get(struct in_addr *pptr)
 {
     PyErr_SetString(PyExc_NotImplementedError, "Getting in_addr");