VM VirtualBox without antivirus in host with Avast Internet installed

Hi, my problem is this.
I have a recent computer with Win10 64bit OS, in which I have mounted a VM Virtualbox 5.2 with a Win7 32bit OS.
On the host computer is active Avast Internet Security; on the guest VM, instead, I have opted for the alone firewall of Windows, not to get heavy the VM.
I am right to believe that in the VM is some Troy: how can I scan the VM with Avast on the host?
Or must I also install Avast in the VM? And in this case, there could be conflict among Avast-host and Avast-guest?
I hope to have been clear.
Thanks