block: Move bdrv_truncate() implementation to io.c

This moves the bdrv_truncate() implementation from block.c to block/io.c
so it can have access to the tracked requests infrastructure.

This involves making refresh_total_sectors() public (in block_int.h).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>