block: Add .bdrv_truncate() error messages

block: Add .bdrv_truncate() error messages

Add missing error messages for the block driver implementations of
.bdrv_truncate(); drop the generic one from block.c's bdrv_truncate().

Since one of these changes touches a mis-indented block in
block/file-posix.c, this patch fixes that coding style issue along the
way.

Signed-off-by: Max Reitz <[email protected]>
Message-id: 20170328205129[email protected]
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Max Reitz <[email protected]>