Set context_aware in drive_mirror_stress

The "drive_mirror_stress" uses "error_context" without setting
"context_aware" which leads to error. Let's set it.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/drive_mirror_stress.py

@@ -75,6 +75,7 @@ class DriveMirrorStress(drive_mirror.DriveMirror):
             offset = _offset
 
 
+@error_context.context_aware
 def run(test, params, env):
     """
     drive_mirror_stress test: