cpu: fix possible crash in getModels

cpu: fix possible crash in getModels

Commit 86a15a25 introduced a new cpu driver API 'getModels'. Public API
allow you to pass NULL for models to get only number of existing models.
However the new code will crash with segfault so we have to count with
the possibility that the user wants only the number.

There is also difference in order of the models gathered by this new API
as the old approach was inserting the elements to the end of the array
so we should use 'VIR_APPEND_ELEMENT'.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>