libxl: Avoid possible use of uninitialized mem in libxlDomainStart

The 'libxl_domain_config' object is stack allocated which means its
memory contents are undefined. The libxl_domain_config_dispose() call
is only safe if the memory is initialized to a defined state. Not all
code paths which reach libxl_domain_config_dispose() will ensure that
libxl_domain_config_init() is called. Move the libxl_domain_config_init()
call earlier in the function to ensure all codepaths have defined
memory state.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/libxl/libxl_domain.c

@@ -1100,6 +1100,8 @@ libxlDomainStart(libxlDriverPrivatePtr driver, virDomainObjPtr vm,
 #endif
     virHostdevManagerPtr hostdev_mgr = driver->hostdevMgr;
 
+    libxl_domain_config_init(&d_config);
+
     if (libxlDomainObjPrivateInitCtx(vm) < 0)
         return ret;
 
@@ -1149,8 +1151,6 @@ libxlDomainStart(libxlDriverPrivatePtr driver, virDomainObjPtr vm,
         VIR_FREE(managed_save_path);
     }
 
-    libxl_domain_config_init(&d_config);
-
     if (libxlBuildDomainConfig(driver->reservedVNCPorts, vm->def,
                                priv->ctx, &d_config) < 0)
         goto endjob;