kvm tools: Simply write_in_full() check semantics

kvm tools: Simply write_in_full() check semantics

write_in_full() would not return until count byes has been written or
error has occurred.

So

   if (write_in_full(fd, buf, count) < 0)
	goto err;

is enough.

And

   if (write_in_full(fd, buf, count) != count)
	goto err;

is not necessary.

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>