Fix wrong region_offset when overlaying a page with another

cpu_register_physical_memory_log() does not update region_offset
if a page was previously registered for the same address.  This
could cause mmio accesses going to the wrong place, by using the
old region_offset.
Signed-off-by: NAvi Kivity <avi@redhat.com>
Acked-by: NAndreas Färber <afaerber@suse.de>
Reviewed-by: NRichard Henderson <rth@twiddle.net>

exec.c

@@ -2542,6 +2542,7 @@ void cpu_register_physical_memory_log(target_phys_addr_t start_addr,
                 p->region_offset = 0;
             } else {
                 p->phys_offset = phys_offset;
+                p->region_offset = region_offset;
                 if ((phys_offset & ~TARGET_PAGE_MASK) <= IO_MEM_ROM ||
                     (phys_offset & IO_MEM_ROMD))
                     phys_offset += TARGET_PAGE_SIZE;