block: add bdrv_add_before_write_notifier()

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>