security, apparmor: implement domainSetPathLabel
This came up in discussions around huge pages, but it will cover
more per guest paths that should be added to the guests apparmor profile:
- keys via qemuDomainWriteMasterKeyFile
- per domain dirs via qemuProcessMakeDir
- memory backing paths via qemuProcessBuildDestroyMemoryPathsImpl
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>