xl: change default dump directory

Upstream xl uses /var/xen/dump/ as the default directory for
domain core dumps.  This patch changes the path to the more
FHS-friendly location of /var/lib/xen/dump/, which was also
the path used in SLE11.

See bsc#900292
Index: xen-4.5.0-testing/tools/libxl/xl_cmdimpl.c
===================================================================
--- xen-4.5.0-testing.orig/tools/libxl/xl_cmdimpl.c
+++ xen-4.5.0-testing/tools/libxl/xl_cmdimpl.c
@@ -2092,7 +2092,7 @@ static int handle_domain_death(uint32_t
         char *corefile;
         int rc;
 
-        if (asprintf(&corefile, "/var/xen/dump/%s", d_config->c_info.name) < 0) {
+        if (asprintf(&corefile, "/var/lib/xen/dump/%s", d_config->c_info.name) < 0) {
             LOG("failed to construct core dump path");
         } else {
             LOG("dumping core to %s", corefile);