qemu-nbd: properly report error if qemu_daemon() is failed

qemu-nbd: properly report error if qemu_daemon() is failed

errno has been overwritten by dup2() just below qemu_daemon() and thus
improperly returned to the caller. Fix accordingly.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20230717145544.194786-5-den@openvz.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: reorder patch series]
Signed-off-by: Eric Blake <eblake@redhat.com>