zdtm: Handle --external option in RPC

Travis sets up the GCOV variable to collect coverage and
provides an external mount for this. Thus handling this
option in RPC mode is essential.

travis-ci: success for tests: Add RPC testing to CI (rev2)
Acked-by: Andrey Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm.py

@@ -713,6 +713,9 @@ class criu_rpc:
 			if arg == '--root':
 				criu.opts.root = args.pop(0)
 				continue
+			if arg == '--external':
+				criu.opts.external.append(args.pop(0))
+				continue
 
 			raise test_fail_exc('RPC for %s required' % arg)