um: Remove unsafe printks from the io thread

Printk out of the io thread has been proven to be unsafe. This
is not surprising as the thread is part of the UML hypervisor
code. It is not supposed to invoke any kernel code/resources.

It is necesssary to pass the error to the block IO layer and let it

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>