qcow2: fix flushing after dirty bitmap metadata writes

qcow2: fix flushing after dirty bitmap metadata writes

update_header_sync itself does not need to flush the caches to disk.
The only paths that allocate clusters are:

- bitmap_list_store with in_place=false, called by update_ext_header_and_dir

- store_bitmap_data, called by store_bitmap

- store_bitmap, called by qcow2_store_persistent_dirty_bitmaps and
  followed by update_ext_header_and_dir

So in the end the central place where we need to flush the caches
is update_ext_header_and_dir.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>