virJSONValueCopy: Don't use virJSONValue(Object|Array)Append

virJSONValueCopy: Don't use virJSONValue(Object|Array)Append

We know the exact number of keys or array members for the copied objects
so we can pre-allocate the arrays rather than inserting into them in a
loop incurring realloc copy penalty.

Also virJSONValueCopy now can't fail since all of the functions
allocating the different cases use just g_new/g_strdup internally so we
can remove the NULL checks from the recursive calls.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>