loader/arm64/xen_boot: Remove correctly all modules loaded by xen_module command

loader/arm64/xen_boot: Remove correctly all modules loaded by xen_module command

We need to use FOR_LIST_ELEMENTS_SAFE() instead of FOR_LIST_ELEMENTS()
as single_binary_unload(), called during the loop, is changing the list
using grub_list_remove(). Given the environment probably the old code
simply removed only the first module on the list not freeing all the others.

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>