A future patch will be using a 'name':{dictionary} entry in the
QAPI schema to specify a default value for an optional argument
(see previous commit message for more details why); but existing
use of inline nested structs conflicts with that goal. This patch
fixes one of only two commands relying on nested types, by
breaking the nesting into an explicit type; it means that the
type is now boxed instead of unboxed in C code, but the QMP wire
format is unaffected by this change.

Prefer the safer g_new0() while making the conversion.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>