This switches qemu-nbd from a QemuOpts-based parser for --object to
user_creatable_process_cmdline() which uses a keyval parser and enforces
the QAPI schema.

Apart from being a cleanup, this makes non-scalar properties accessible.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>