hmp: Drop pointless allocation during qapi visit

hmp: Drop pointless allocation during qapi visit

The qapi visitor contract allows us to visit a virtual structure,
where we don't have any corresponding qapi struct.  Most such uses
pass NULL for @obj; but these two callers were passing a dummy
pointer, which then gets allocated to heap memory but then
immediately freed without use.  Clean this up to suppress unwanted
allocation, like we do elsewhere.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1454075341-13658-7-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>