qemu-option: Guard against qemu_opts_set_defaults() misuse

Commit 6d4cd40 fixed qemu_opts_set_defaults() for an existing corner
case, but broke it for another one that can't be reached in current
code.

Quote from its commit message:

    I believe [opts_parse()] attempts to do the following:

        If options don't yet exist, create new options
        Else, if defaults, modify the existing options
        Else, if list->merge_lists, modify the existing options
        Else, fail

The only caller that passes true for defaults is
qemu_opts_set_defaults().

The commit message then claims:

    A straightforward call of qemu_opts_create() does exactly that.

Wrong.  When !list->merge_lists, and the option string doesn't contain
id=, and options without ID exist, then we don't actually modify the
existing options, we create new ones.

Not reachable, because we never pass lists with !list->merge_lists to
qemu_opts_set_defaults().

Guard against possible (if unlikely) future misuse with assert().

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1375428840-5275-1-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
