zdtm.py: catch KeyError instead of ValueError for sets

In [3]: a
Out[3]: {1, 2, 3}

In [4]: a.remove(5)
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-4-af76ca25878a> in <module>()
----> 1 a.remove(5)

KeyError: 5
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm.py

@@ -977,7 +977,7 @@ def run_tests(opts):
 			if not criu_cli.check("userns"):
 				try:
 					run_flavs.remove("uns")
-				except ValueError:
+				except KeyError:
 					# don't worry if uns isn't in run_flavs
 					pass