bhyve: Conditionally compile framebuffer-related code

[markj]

Summary

The arm64 port does not implement VGA, so move the device model sources.
Compile framebuffer code only on amd64 for now, but do not move the
sources, as we ought to be able to add support later.

No functional change intended.

[corvink]

Is VGA supported on arm64 or not? While reading the commit message it sounds like missing VGA support is only a limitation of our arm64 port. If it's just a limitation or our port, we shouldn't move those files.

[jhb]

I kind of doubt there's real VGA on arm64, just different frame buffer drivers that probably don't provide a legacy VGA interface.

[markj]

In D40740#927299, @corvink wrote:

Is VGA supported on arm64 or not? While reading the commit message it sounds like missing VGA support is only a limitation of our arm64 port. If it's just a limitation or our port, we shouldn't move those files.

Indeed, I wasn't able to turn up any notion of a native VGA driver on arm64. The software interface uses I/O ports so can't be directly implemented on arm64. I'll reword the commit log message to make this more clear, but I'm fairly sure we can safely move this file to amd64/.