test: Disable packagekit in TestStorageLegacyVDO

Creating a VDO needs oodles of memory, almost as much as our standard
VMs provide. packagekit often starts up in parallel and does its own
CPU/memory hungry thing, both together then collide and OOM.

We don't need packagekit for this test, so disable it like we did in so
many others already.

Cherry-picked from main commit 172dd31.

test/verify/check-storage-vdo

@@ -175,6 +175,11 @@ class TestStorageVDO(storagelib.StorageCase):
 @testlib.onlyImage("legacy VDO API only supported on RHEL 8", "rhel-8*", "centos-8*")
 class TestStorageLegacyVDO(storagelib.StorageCase):
 
+    def setUp(self):
+        super().setUp()
+        # packagekit eats too much CPU/memory in the background
+        self.machine.execute("systemctl mask packagekit; systemctl stop packagekit")
+
     def testVdo(self):
         m = self.machine
         b = self.browser