Add missing 'const' annotations for internal domain conf helpers

Add missing 'const' annotations for internal domain conf helpers

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 *