Undocumented image.os behavior for Windows

[foxtrotcz]

So thanks to reading this pull request for solving RTC base in Windows VM I found out you configure QEMU differently if image.os contains word windows. For example you disable 9p and vsock.

This is quite important but I dont see it documented anywhere. Regular users will set up Windows VM according to Simos guide and will never figure out they should set image.os.
I didnt know it until now.

This can cause wrong behavior of Windows VM in Incus simply because users dont know they should set image.os: Windows.

I dont know if this is the only config that does something like this, but I believe it should be clearly documented so people know they should set it.