qemu-nbd: fix regression with qemu-nbd --fork run over ssh

qemu-nbd: fix regression with qemu-nbd --fork run over ssh

Commit e6df58a5578fee7a50bbf36f4a50a2781cff855d
    Author: Hanna Reitz <hreitz@redhat.com>
    Date:   Wed May 8 23:18:18 2019 +0200
    qemu-nbd: Do not close stderr

has introduced an interesting regression. Original behavior of
    ssh somehost qemu-nbd /home/den/tmp/file -f raw --fork
was the following:
 * qemu-nbd was started as a daemon
 * the command execution is done and ssh exited with success

The patch has changed this behavior and 'ssh' command now hangs forever.

According to the normal specification of the daemon() call, we should
endup with STDERR pointing to /dev/null. That should be done at the
very end of the successful startup sequence when the pipe to the
bootstrap process (used for diagnostics) is no longer needed.

This could be achived in the same way as done for 'qemu-nbd -c' case.
That was commit 0eaf453e, also fixing up e6df58a5. STDOUT copying to
STDERR does the trick.

This also leads to proper 'ssh' connection closing which fixes my
original problem.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
CC: Hanna Reitz <hreitz@redhat.com>
CC: <qemu-stable@nongnu.org>
Message-ID: <20230717145544.194786-3-den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>