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block: add bdrv_add_before_write_notifier()
The bdrv_add_before_write_notifier() function installs a callback that
is invoked before a write request is processed. This will be used to
implement copy-on-write point-in-time snapshots where we need to copy
out old data before overwriting it.
Note that BdrvTrackedRequest is moved to block_int.h since it is passed
to .notify() functions.
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>