block: call the snapshot handlers of the protocol drivers

block: call the snapshot handlers of the protocol drivers

When snapshot handlers are not defined in the format driver, it is
better to call the ones of the protocol driver.  This enables us to
implement snapshot support in the protocol driver.

We need to call bdrv_close() and bdrv_open() handlers of the format
driver before and after bdrv_snapshot_goto() call of the protocol.  It is
because the contents of the block driver state may need to be changed
after loading vmstate.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>