exec: fix write tlb entry misused as iotlb

A couple of code paths check the lower bits of CPUTLBEntry::addr_write
against io_mem_ram as a way of looking for a dirty RAM page.  This works
by accident since the value is zero, which matches all clear bits for
TLB_INVALID, TLB_MMIO, and TLB_NOTDIRTY (indicating dirty RAM).

Make it work by design by checking for the proper bits.

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