libxl/hotplug: add support for getting domid

This patch writes Dom0 domid on xenstore (like it's done for other
guests), and adds a libxl helper function to fetch that domid from
xenstore.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
[ ijc -- dropped xencommons hunk, same change was made independently
  in 02ebea7768 ]

tools/libxl/libxl.c

@@ -1692,6 +1692,23 @@ int libxl_vncviewer_exec(libxl_ctx *ctx, uint32_t domid, int autopass)
     return ERROR_FAIL;
 }
 
+int libxl__get_domid(libxl__gc *gc, uint32_t *domid)
+{
+    int rc;
+    const char *xs_domid;
+
+    rc = libxl__xs_read_checked(gc, XBT_NULL, DOMID_XS_PATH, &xs_domid);
+    if (rc || !xs_domid) {
+        rc = rc ? rc : ERROR_FAIL;
+        goto out;
+    }
+
+    *domid = atoi(xs_domid);
+
+out:
+    return rc;
+}
+
 /******************************************************************************/
 
 /* generic callback for devices that only need to set ao_complete */

tools/libxl/libxl_internal.h

@@ -101,6 +101,7 @@
 #define STUBDOM_SPECIAL_CONSOLES 3
 #define TAP_DEVICE_SUFFIX "-emu"
 #define DISABLE_UDEV_PATH "libxl/disable_udev"
+#define DOMID_XS_PATH "domid"
 
 #define ARRAY_SIZE(a) (sizeof(a) / sizeof(a[0]))
 
@@ -978,6 +979,8 @@ _hidden const char *libxl__device_nic_devname(libxl__gc *gc,
                                               uint32_t devid,
                                               libxl_nic_type type);
 
+_hidden int libxl__get_domid(libxl__gc *gc, uint32_t *domid);
+
 /*
  * libxl__ev_devstate - waits a given time for a device to
  * reach a given state.  Follows the libxl_ev_* conventions.