Improve XendCheckpoint's forkHelper error handling.

Improve XendCheckpoint's forkHelper error handling.

When inputHandler raises an exception, close stdin and stdout in the
child before attempting to close stderr, which is held open by a
separate thread. Without this patch, exceptions raised by device
migration caused xend to hang on child.childerr.close().

Signed-off-by: Brendan Cully <brendan@cs.ubc.ca>