libxl: ensure xs transaction is initialised in libxl__device_pci_add_xenstore

Run "xl pci-attach <domain> <pci_device>", the 2nd time fails:
xl: libxl_xshelp.c:209: libxl__xs_transaction_start: Assertion `!*t' failed.
Aborted

To fix that, initialize xs_transaction to avoid libxl__xs_transaction_start
assertion error.

Signed-off-by: Chunyan Liu <cyliu@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- updated commit message ]

(cherry picked from commit 4600d75604)

tools/libxl/libxl_pci.c

@@ -123,7 +123,7 @@ static int libxl__device_pci_add_xenstore(libxl__gc *gc, uint32_t domid, libxl_d
     flexarray_t *back;
     char *num_devs, *be_path;
     int num = 0;
-    xs_transaction_t t;
+    xs_transaction_t t = XBT_NULL;
     libxl__device *device;
     int rc;
     libxl_domain_config d_config;