exec: limit system memory size

The page table logic in exec.c assumes
that memory addresses are at most TARGET_PHYS_ADDR_SPACE_BITS.

But pci addresses are full 64 bit so if we try to render them ignoring
the extra bits, we get strange effects with sections overlapping each
other.

To fix, simply limit the system memory size to
 1 << TARGET_PHYS_ADDR_SPACE_BITS,
pci addresses will be rendered within that.

Cc: qemu-stable@nongnu.org
Reported-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
