Fix up pxe boot (Glauber Costa)

Fix up pxe boot (Glauber Costa)

As discussed in
http://lists.gnu.org/archive/html/qemu-devel/2008-08/msg00667.html,
current pxe boot is broken for some use cases. The problem
goes away if we reduce the number of allowed bits in the address space
to 32 (which has the side effect of reducing guest max mem size to 4Gb).

After digging for a while, it turns out that it happens because pxelinux
tries to access address 0x10009e9a6, which does not fit a 32-bit address.
A closer look, however, reveals this access is totally valid: It's just
0x9e9a6 with an add carry.

To avoid this, this patch casts the address passed to the POPL macro to
a 32-bit value. This is also done, although just theorectically, for
PUSHL too.

Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Reported-by: Chris Lalancette <clalance@redhat.com>
CC: Eduardo Habkost <ehabkost@redhat.com>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5182 c046a42c-6fe2-441c-8c8c-71466251a162