Allocate physical memory in low virtual address space

KVM on S390x requires the virtual address space of the guest's RAM to be
within the first 256GB.

The general direction I'd like to see KVM on S390 move is that this requirement
is losened, but for now that's what we're stuck with.

So let's just hack up qemu_ram_alloc until KVM behaves nicely :-).

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
