block: add flush callback
This patch adds callback for flush request. This callback is responsible for flushing whole block devices stack. bdrv_flush function does not proceed to underlying devices. It should be performed by this callback function, if needed. Signed-off-by: NPavel Dovgalyuk <pavel.dovgaluk@ispras.ru> Signed-off-by: NKevin Wolf <kwolf@redhat.com>
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