lxc/completion: Add missing completions for lxc config unset

Signed-off-by: Kadin Sayani <[email protected]>

lxc/completion.go

@@ -371,9 +371,12 @@ func (g *cmdGlobal) cmpInstanceSetKeys(instanceName string) ([]string, cobra.She
 	configKeys := make([]string, 0, keyCount)
 
 	for configKey := range instance.Config {
-		// We only want to return the intersection between allInstanceConfigKeys and configKeys to avoid returning the full instance config.
+		// We only want to return the intersection between allInstanceConfigKeys
+		// and configKeys to avoid returning the full instance config.
+		// We also need to account for user defined keys such as user.* and linux.sysctl.*.
 		_, exists := keySet[configKey]
-		if exists {
+		if exists || strings.HasPrefix(configKey, "user.") ||
+			strings.HasPrefix(configKey, "linux.sysctl.") {
 			configKeys = append(configKeys, configKey)
 		}
 	}