The primary surface, i.e. qxl->primary, the only surface with id==0, is
allocated in qxl_surface_cache_create_primary with prev==next==NULL.
Unlinking it was producing a wrong cache->free_surfaces == NULL. This
was not a problem because unlinking the primary only happened in
switch_host, which then called surface_cache_init. In a following commit
switch_host is simplified to destroy-primary+create-primary, so this bug
needs to be fixed first to avoid leaking surfaces and reaching a no
surface available situation.