libxl: fix AttachDeviceConfig on hostdev type

After attach-device a <hostdev> with --config, new device doesn't
show up in dumpxml and in guest.

To fix that, set dev->data.hostdev = NULL after work so that the
pointer is not freed, since vmdef has the pointer and still need it.
Signed-off-by: NChunyan Liu <cyliu@suse.com>
(cherry picked from commit 56945e13)

src/libxl/libxl_driver.c

@@ -3312,6 +3312,7 @@ libxlDomainAttachDeviceConfig(virDomainDefPtr vmdef, virDomainDeviceDefPtr dev)
 
             if (virDomainHostdevInsert(vmdef, hostdev) < 0)
                 return -1;
+            dev->data.hostdev = NULL;
             break;
 
         default: