conf: Utilize more of VIR_(APPEND|INSERT|DELETE)_ELEMENT

conf: Utilize more of VIR_(APPEND|INSERT|DELETE)_ELEMENT

This fixes a possible double free. In virNetworkAssignDef() if
virBitmapNew() fails, then virNetworkObjFree(network) is called.
However, with network->def pointing to actual @def. So if caller
frees @def again, ...

Moreover, this fixes one possible memory leak too. In
virInterfaceAssignDef() if appending to the list of interfaces
fails, we ought to call virInterfaceObjFree() instead of bare
VIR_FREE().

Although, in order to do that some array size variables needs
to be turned into size_t rather than int.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>