qemu-nbd: Fix invalid usage of the first argument of errx

errx takes the exit status of a process as the first
argument. Passing errno to it is wrong. Instead the
patch lets errx take EXIT_FAILURE.

Signed-off-by: Ryota Ozaki <ozaki.ryota@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
