Do not fail when kpartx is not available

We don't need to fail just because kpartx is missing, we can just
log warning and not activate the partitions.

blivet/devices/dm.py

@@ -127,13 +127,20 @@ def get_dm_node(self):
 
     def setup_partitions(self):
         log_method_call(self, name=self.name)
+        if not availability.KPARTX_APP.available:
+            log.warning("'kpartx' not available, not activating partitions on %s", self.path)
+            return
+
         rc = util.run_program(["kpartx", "-a", "-s", self.path])
         if rc:
             raise errors.DMError("partition activation failed for '%s'" % self.name)
         udev.settle()
 
     def teardown_partitions(self):
         log_method_call(self, name=self.name)
+        if not availability.KPARTX_APP.available:
+            log.warning("'kpartx' not available, not deactivating partitions on %s", self.path)
+            return
         rc = util.run_program(["kpartx", "-d", "-s", self.path])
         if rc:
             raise errors.DMError("partition deactivation failed for '%s'" % self.name)