exec: fix code tlb entry misused as iotlb in get_page_addr_code()

exec: fix code tlb entry misused as iotlb in get_page_addr_code()

get_page_addr_code() reads a code tlb entry, but interprets it as an
iotlb entry.  This works by accident since the low bits of a RAM code
tlb entry are clear, and match a RAM iotlb entry.  This accident is
about to unhappen, so fix the code to use an iotlb entry (using the
code entry with TLB_MMIO may fail if the page is a watchpoint).

Signed-off-by: Avi Kivity <avi@redhat.com>