With all externally visible functions changed to use BlockBackend, this
patch makes nbd use BlockBackend for everything internally as well.

While touching them, substitute 512 by BDRV_SECTOR_SIZE in the calls to
blk_read(), blk_write() and blk_co_discard().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1416309679-333-6-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>