target-i386: Mask NX bit from cpu_get_phys_page_debug result

This was a long pending bug, now revealed by the assert in
phys_page_find that stumbled over the large page index returned by
cpu_get_phys_page_debug for NX-marked pages: We need to mask out NX and
all user-definable bits 52..62 from PDEs and the final PTE to avoid
corrupting physical addresses.

Reviewed-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>