Fix bdev unittest when run under distcheck

The path to the filename for drbd8 proc data is not correctly computed
when using distcheck. The patch duplicates it from the other drbd tests.

Reviewed-by: ultrotter

test/ganeti.bdev_unittest.py

@@ -115,7 +115,11 @@ class TestDRBD8Status(unittest.TestCase):
 
   def setUp(self):
     """Read in txt data"""
-    self.proc_data = bdev.DRBD8._GetProcData(filename="data/proc_drbd8.txt")
+    proc_data = "data/proc_drbd8.txt"
+    prefix = os.environ.get("srcdir", None)
+    if prefix:
+      proc_data = prefix + "/" + proc_data
+    self.proc_data = bdev.DRBD8._GetProcData(filename=proc_data)
     self.mass_data = bdev.DRBD8._MassageProcData(self.proc_data)
 
   def testMinorNotFound(self):