The virDomainNetGetActualBridgeName and virDomainNetGetActualDirectDev
methods both return strings that point to data in the virDomainDefPtr
struct, and should therefore not be freed. The return values should
thus be 'const char *' not 'char *'.

* src/conf/domain_conf.c, src/conf/domain_conf.h: Mark const
* src/network/bridge_driver.c: Update to use a const char *