util: alloc: Introduce freeing helpers that clear the memory before freeing

util: alloc: Introduce freeing helpers that clear the memory before freeing

For a few cases where we handle secret information it's good to clear
the buffers containing sensitive data before freeing them.

Introduce VIR_DISPOSE, VIR_DISPOSE_N and VIR_DISPOSE_STRING that allow
simple clearing fo the buffers holding sensitive information on cleanup
paths.