block: Extract common write req handling

block: Extract common write req handling

As a mechanical refactoring patch, this is the first step towards
unified and more correct write code paths. This is helpful because
multiple BlockDriverState fields need to be updated after modifying
image data, and it's hard to maintain in multiple places such as copy
offload, discard and truncate.

Suggested-by: Kevin Wolf <[email protected]>
Signed-off-by: Fam Zheng <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>