I already understand that I should download a right System image to use Android virtual device.
And it works fine on SDK24(7.0). I downloaded SDK 24 and Intel x86 Atom_64 System Image. I am able to create SDK 24 AVD and works fine.
But I tried same with SDK25. I downloaded SDK 25 and Google APIs Intel x86 Atom_64 System Image. But when I try to create AVD by selecting Device on the list and set Target as "Android 7.1.1 - API Level 25"
Then, CPU/ABI section says "No system images installed for this target". I tried to change 'Device' on the list.
None of them works.
I have the exact same issue.
My solution : I installed Android Studio and have it pointing to my Xamarin SDK location. I use Android Studio to create and start my emulators. Visual Studio for Mac is quite happy to deploy to them, once they are running. It seems to be a bug in VS4Mac. I also cheat and use Android Studio to do my XML in and then copy & paste it to VS4Mac.