virnetdev.c: Use g_auto*()

While I'm at it, use more g_autofree and g_autoptr() in this
file. This also fixes a possible mem-leak in
virNetDevGetVirtualFunctions().
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>