util: Introduce VIR_FREE
util: Introduce VIR_FREE

Just like libvirt has it. After freeing pointer set it to NULL to
avoid double frees.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Erik Skultety <2c14d38fa47c8799f1b9c16280abe27f8edfec6e@redhat.com>

zippy2/libvirt-php,zippy2/libvirt-php,libvirt/libvirt-php,libvirt/libvirt-php,libvirt/libvirt-php,zippy2/libvirt-php