target-i386: Fix mask of pte index in memory mapping

Function walk_pte() needs pte index to calculate virtual address.
However, pte index of PAE paging or IA-32e paging is 9 bit, so the mask
should be 0x1ff.

Signed-off-by: Qiao Nuohan <qiaonuohan@cn.fujitsu.com>
Reviewed-by: Jesse Larrew <jlarrew@linux.vnet.ibm.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>