In order to open a BDS which inherits a BB's root state,
blk_get_open_flags_from_root_state() is used to inquire the flags to be
passed to bdrv_open(), and blk_apply_root_state() is used to apply the
remaining state after the BDS has been opened.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>