Rather than open-coding the translation from the deprecated
NbdServerAddOptions type to the preferred BlockExportOptionsNbd, it's
better to utilize QAPI_CLONE_MEMBERS. This solves a couple of issues:
first, if we do any more refactoring of the base type (which an
upcoming patch plans to do), we don't have to revisit the open-coding.
Second, our assignment to arg->name is fishy: the generated QAPI code
currently does not visit it if arg->has_name is false, but if it DID
visit it, we would have introduced a double-free situation when arg is
finally freed.
Signed-off-by: Eric Blake<eblake@redhat.com>