blockdev-nbd: Boxed argument type for nbd-server-add

Move the arguments of nbd-server-add to a new struct BlockExportNbd and
convert the command to 'boxed': true. This makes it easier to share code
with the storage daemon.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200224143008.13362-11-kwolf@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>