block: Allow error return in BlockDevOps.change_media_cb()

block: Allow error return in BlockDevOps.change_media_cb()

Some devices allow a media change between read-only and read-write
media. They need to adapt the permissions in their .change_media_cb()
implementation, which can fail. So add an Error parameter to the
function.

Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Acked-by: Fam Zheng <[email protected]>