Document setting up shared folders between the host and guest VM

It would be useful if [`docs/KVM.md`](https://github.com/Fmstrat/winapps/blob/main/docs/KVM.md) had some guidance on setting up a shared folder, as is done in VirtualBox. This allows for more seamless interoperability between the host and guest.

By the way, thanks for providing this project! It's working well so far. I'm trying to evaluate it to use MSVC 2019 on a project stored on the host's filesystem. Right now, I'm using VirtualBox for this purpose which is less CPU-efficient compared to QEMU/KVM.