update_payload: Add XZ compression support

This patch adds support for checking a payload that has REPLACE_XZ
operations. REPLACE_XZ was added in minor version 3.

BUG=chromium:758792
TEST=unittests pass; paycheck.py with a xz generated payload pass;
CQ-DEPEND=CL:823234

Change-Id: If82c767a201b189e464f459d5c19485e5278d9b1
Reviewed-on: https://chromium-review.googlesource.com/823227
Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>
diff --git a/scripts/update_payload/test_utils.py b/scripts/update_payload/test_utils.py
index 61a91f5..bfdc40e 100644
--- a/scripts/update_payload/test_utils.py
+++ b/scripts/update_payload/test_utils.py
@@ -282,7 +282,7 @@
 
     Args:
       is_kernel: whether this is a kernel (True) or rootfs (False) operation
-      op_type: one of REPLACE, REPLACE_BZ, MOVE or BSDIFF
+      op_type: one of REPLACE, REPLACE_BZ, REPLACE_XZ, MOVE or BSDIFF
       src_extents: list of (start, length) pairs indicating src block ranges
       src_length: size of the src data in bytes (needed for BSDIFF)
       dst_extents: list of (start, length) pairs indicating dst block ranges