target-i386: fix abort on bad PML4E/PDPTE/PDE/PTE addresses

target-i386: fix abort on bad PML4E/PDPTE/PDE/PTE addresses

The code used to walk IA-32e page-tables, and possibly PAE page-tables,
uses the bit mask ~0xfff to get the next PML4E/PDPTE/PDE/PTE address.

However, as we use a uint64_t to store the resulting address, that mask
gets expanded to 0xfffffffffffff000 which not only ends up selecting
reserved bits but also selects the XD bit (execute-disable) which
happens to be enabled by Windows 8, causing qemu_get_ram_ptr() to abort.

This commit fixes that problem by replacing ~0xfff by a correct mask
that only selects the address bit range (ie. bits 51:12).

Signed-off-by: Luiz Capitulino <[email protected]>
Reviewed-by: Laszlo Ersek <[email protected]>