update_engine: Add fds for the source partitions.

Add new fds for the source partition, one for the rootfs and another for
the kernel. These are opened if we have a delta update with minor
version 2.

This change also adds support for changing the minor versions in tests.
There is a new private member, supported_minor_version_, which defaults
to kSupportedMinorPayloadVersion. It is set in the unit tests with calls
to SetSupportedVersion.

BUG=chromium:463573
TEST=`FEATURES=test emerge-link update_engine`

Change-Id: Ib988c91eb450b2499c615ae65b271691dfd9c651
Reviewed-on: https://chromium-review.googlesource.com/260950
Trybot-Ready: Allie Wood <alliewood@chromium.org>
Tested-by: Allie Wood <alliewood@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Allie Wood <alliewood@chromium.org>
diff --git a/filesystem_copier_action_unittest.cc b/filesystem_copier_action_unittest.cc
index 51eb318..1aa1065 100644
--- a/filesystem_copier_action_unittest.cc
+++ b/filesystem_copier_action_unittest.cc
@@ -330,7 +330,7 @@
 
   ObjectFeederAction<InstallPlan> feeder_action;
   const char* kUrl = "http://some/url";
-  InstallPlan install_plan(false, true, kUrl, 0, "", 0, "", "", "", "");
+  InstallPlan install_plan(false, true, kUrl, 0, "", 0, "", "", "", "", "", "");
   feeder_action.set_obj(install_plan);
   FilesystemCopierAction copier_action(&fake_system_state_, false, false);
   ObjectCollectorAction<InstallPlan> collector_action;
@@ -364,6 +364,8 @@
                            "",
                            "/no/such/file",
                            "/no/such/file",
+                           "/no/such/file",
+                           "/no/such/file",
                            "");
   feeder_action.set_obj(install_plan);
   FilesystemCopierAction copier_action(&fake_system_state_, false, false);