tests: hide stderr for postcopy recovery test

tests: hide stderr for postcopy recovery test

We dumped something when network failure happens.  We should avoid those
messages to be dumped when running the tests:

  $ ./tests/migration-test -p /x86_64/migration/postcopy/recovery
  /x86_64/migration/postcopy/recovery: qemu-system-x86_64: check_section_footer: Read section footer failed: -5
  qemu-system-x86_64: Detected IO failure for postcopy. Migration paused.
  qemu-system-x86_64: Detected IO failure for postcopy. Migration paused.
  OK

After the patch:

  $ ./tests/migration-test -p /x86_64/migration/postcopy/recovery
  /x86_64/migration/postcopy/recovery: OK

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180710091902.28780-11-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>