s390x: complain when allocating ram fails

While trying out the > 64GB guest RAM patch, I hit some virtual address
limitations of my host system, which resulted in mmap failing. Unfortunately,
qemu didn't tell me about this failure, but just used the NULL pointer
happily, resulting in either segmentation faults or other fun errors.

To spare other users from tracing this down, let's print a nice message
instead so the user can figure out what's wrong from there.

Signed-off-by: Alexander Graf <agraf@suse.de>
