block/copy-before-write: introduce cbw_init()

block/copy-before-write: introduce cbw_init()

Move part of bdrv_cbw_append() to new function cbw_open(). It's an
intermediate step for adding normal .bdrv_open() handler to the
filter. With this commit no logic is changed, but we have a function
which will be turned into .bdrv_open() handler in future commit.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210824083856.17408-15-vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>