migration: Plug memory leak in migrate-set-cache-size command

We call g_free() after cache_fini() in migration_end(), but we don't
call it after cache_fini() in xbzrle_cache_resize(), leaking the
memory.

cache_init() and cache_fini() are a pair.  Since cache_init()
allocates the cache, let cache_fini() free it.  This plugs the leak.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
