qemu-nbd: improve error message for dup2 error

qemu-nbd: improve error message for dup2 error

This error happens if we are not able to close the pipe to the
parent (to trace errors in the child process) and assign stderr to
/dev/null as required by the daemonizing convention.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Suggested-by: Eric Blake <eblake@redhat.com>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20230906093210.339585-2-den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: commit message grammar]
Signed-off-by: Eric Blake <eblake@redhat.com>