Use the ATTRIBUTE_NONNULL annotation to mark some virConnectPtr
args as mandatory non-null so the compiler can warn of mistakes

* src/conf/domain_event.h: All virConnectPtr args must be non-null
* src/qemu/qemu_conf.h: qemudBuildCommandLine and
  qemudNetworkIfaceConnect() must be given non-null connection
* tests/qemuxml2argvtest.c: Provide a non-null (dummy) connection to
  qemudBuildCommandLine()