block: Pull out bdrv_default_perms_for_cow()

block: Pull out bdrv_default_perms_for_cow()

Right now, bdrv_format_default_perms() is used by format parents
(generally). We want to switch to a model where most parents use a
single BdrvChildClass, which then decides the permissions based on the
child role. To do so, we have to split bdrv_format_default_perms() into
separate functions for each such role.

Signed-off-by: Max Reitz <[email protected]>
Message-Id: <20200513110544[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>