test/assembler: use new loop_for_fd helper

Use the new `loop_for_fd` helper that got added to `LoopControl`,
which is basically a version of the existing code.

test/run/test_assemblers.py

@@ -3,7 +3,6 @@
 #
 
 import contextlib
-import errno
 import hashlib
 import json
 import os
@@ -248,26 +247,16 @@ class TestAssemblers(test.TestBase):
 
 @contextlib.contextmanager
 def loop_create_device(ctl, fd, offset=None, sizelimit=None):
-    while True:
+    lo = None
-        lo = loop.Loop(ctl.get_unbound())
-        try:
-            lo.set_fd(fd)
-        except OSError as e:
-            lo.close()
-            if e.errno == errno.EBUSY:
-                continue
-            raise e
-        try:
-            lo.set_status(offset=offset, sizelimit=sizelimit, autoclear=True)
-        except BlockingIOError:
-            lo.clear_fd()
-            lo.close()
-            continue
-        break
     try:
+        lo = ctl.loop_for_fd(fd,
+                             offset=offset,
+                             sizelimit=sizelimit,
+                             autoclear=True)
         yield lo
     finally:
-        lo.close()
+        if lo:
+            lo.close()
 
 
 @contextlib.contextmanager