fixed message for vhci mode only linux. and changed readme

internal/usbip/vhci.go

@@ -166,3 +166,12 @@ func IsVHCIAvailable() bool {
 	_, err := os.Stat(vhciBasePath)
 	return err == nil
 }
+
+// VHCIUnavailableError returns an error describing why VHCI is not available,
+// or nil if VHCI is ready to use.
+func VHCIUnavailableError() error {
+	if IsVHCIAvailable() {
+		return nil
+	}
+	return fmt.Errorf("vhci-hcd Kernel-Modul nicht geladen (ausfuehren: sudo modprobe vhci-hcd)")
+}