qemu-nbd: Use BlockBackend where reasonable

qemu-nbd: Use BlockBackend where reasonable

Because qemu-nbd creates the BlockBackend by itself, it should create
the according BlockDriverState tree by itself as well; that means, it
has call bdrv_open() on its own. This is one of the places where
qemu-nbd still needs to use a BlockDriverState directly (the root BDS
below the BB); other places are the configuration of zero detection
(which may be lifted into the BB eventually, but is not yet) and
temporarily loading a snapshot.

Everywhere else, though, qemu-nbd can and thus should use BlockBackend.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1416309679-333-7-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>