[qemu] Fix reads on unreported memory addresses.

[qemu] Fix reads on unreported memory addresses.
The function cpu_physical_memory_rw() thinks that if an address is
not mmio-related, it is assumed to be a RAM case. This is improper.
When making the assumption, we should make sure the address is less
than the guest physical memory size ram_size.

From: Cui, Dexuan <dexuan.cui@intel.com>
Signed-off-by: Christian Limpach <Christian.Limpach@xensource.com>