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: Chunyan Liu <cyliu@suse.com>