block: Lift some BDS functions to the BlockBackend

block: Lift some BDS functions to the BlockBackend

Create the blk_* counterparts for the following bdrv_* functions (which
make sense to call on the BlockBackend level):
- bdrv_co_write_zeroes()
- bdrv_write_compressed()
- bdrv_truncate()
- bdrv_nb_sectors()
- bdrv_discard()
- bdrv_load_vmstate()
- bdrv_save_vmstate()

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1423162705-32065-2-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>