Fix sign mismatches between public API, driver API and XDR protocol

Fix sign mismatches between public API, driver API and XDR protocol

In most cases this affects flags parameters that are unsigned in the
public and driver API but signed in the XDR protocol. Switch the
XDR protocol to unsigned for those.

A counterexample is virNWFilterGetXMLDesc. Its flags parameter is signed
in the public API and XDR protocol, but unsigned in the driver API.