block: Add BDRV_BLOCK_EOF to bdrv_get_block_status()

block: Add BDRV_BLOCK_EOF to bdrv_get_block_status()

Just as the block layer already sets BDRV_BLOCK_ALLOCATED as a
shortcut for subsequent operations, there are also some optimizations
that are made easier if we can quickly tell that *pnum will advance
us to the end of a file, via a new BDRV_BLOCK_EOF which gets set
by the block layer.

This just plumbs up the new bit; subsequent patches will make use
of it.

Signed-off-by: Eric Blake <[email protected]>
Message-Id: <20170505021500[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Fam Zheng <[email protected]>