virsh: properly interleave shared stdout and stderr

virsh: properly interleave shared stdout and stderr

Without this patch, invoking 'virsh >file 2>&1' results in
error messages appearing before normal output, even if they
occurred later in time than the normal output (since stderr
is unbuffered, but stdout waits until a full buffer).

* tools/virsh.c (print_job_progress, vshError): Flush between
stream transitions.
* tests/undefine: Test it.