page_cache: fix memory leak

XBZRLE encoded migration introduced a MRU page cache
meachnism. Unfortunately, cached items where never freed in
case of a collision in the page cache on cache_insert().

This lead to out of memory conditions during XBZRLE migration
if the page cache was small and there where a lot of collisions
in the cache.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Orit Wasserman <owasserm@redhat.com>

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Juan Quintela <quintela@redhat.com>

page_cache.c

@@ -152,6 +152,9 @@ void cache_insert(PageCache *cache, uint64_t addr, uint8_t *pdata)
     /* actual update of entry */
     it = cache_get_by_addr(cache, addr);
 
+    /* free old cached data if any */
+    g_free(it->it_data);
+
     if (!it->it_data) {
         cache->num_items++;
     }