xcopy: loop over devices using idr helper

This converts the xcopy code to use the idr helper. The next patch
will drop the g_device_list and make g_device_mutex local to the
target_core_device.c file.

Signed-off-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>