xen: create xenstore areas for XenDevice-s

This patch adds a new source module, xen-bus-helper.c, which builds on
basic libxenstore primitives to provide functions to create (setting
permissions appropriately) and destroy xenstore areas, and functions to
'printf' and 'scanf' nodes therein. The main xen-bus code then uses
these primitives [1] to initialize and destroy the frontend and backend
areas for a XenDevice during realize and unrealize respectively.

The 'xen-block' implementation is extended with a 'get_name' method that
returns the VBD number. This number is required to 'name' the xenstore
areas.

NOTE: An exit handler is also added to make sure the xenstore areas are
      cleaned up if QEMU terminates without devices being unrealized.

[1] The 'scanf' functions are actually not yet needed, but they will be
    needed by code delivered in subsequent patches.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony Perard <anthony.perard@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

include/hw/xen/xen-bus-helper.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2018  Citrix Systems Inc.
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#ifndef HW_XEN_BUS_HELPER_H
+#define HW_XEN_BUS_HELPER_H
+
+#include "hw/xen/xen_common.h"
+
+const char *xs_strstate(enum xenbus_state state);
+
+void xs_node_create(struct xs_handle *xsh,  xs_transaction_t tid,
+                    const char *node, struct xs_permissions perms[],
+                    unsigned int nr_perms, Error **errp);
+void xs_node_destroy(struct xs_handle *xsh,  xs_transaction_t tid,
+                     const char *node, Error **errp);
+
+/* Write to node/key unless node is empty, in which case write to key */
+void xs_node_vprintf(struct xs_handle *xsh,  xs_transaction_t tid,
+                     const char *node, const char *key, Error **errp,
+                     const char *fmt, va_list ap)
+    GCC_FMT_ATTR(6, 0);
+void xs_node_printf(struct xs_handle *xsh,  xs_transaction_t tid,
+                    const char *node, const char *key, Error **errp,
+                    const char *fmt, ...)
+    GCC_FMT_ATTR(6, 7);
+
+/* Read from node/key unless node is empty, in which case read from key */
+int xs_node_vscanf(struct xs_handle *xsh,  xs_transaction_t tid,
+                   const char *node, const char *key, Error **errp,
+                   const char *fmt, va_list ap);
+int xs_node_scanf(struct xs_handle *xsh,  xs_transaction_t tid,
+                  const char *node, const char *key, Error **errp,
+                  const char *fmt, ...);
+
+#endif /* HW_XEN_BUS_HELPER_H */