refactor virgl_hw.h

This MR moves resource, caps, and legacy definitions to virgl_hw_{resource,caps,legacy}.h respectively.

The motivation is #175 (closed), where there is a desire to not mix venus definitions with virgl definitions. When venus is merged, its definitions will go into virgl_hw_venus.h. In other words,

• virgl_hw.h: main header with common definitions and include all other headers
  • virgl_hw_legacy.h: legacy definitions
  • virgl_hw_resource.h: resource v1 definitions
  • virgl_hw_caps.h: virgl cap definitions
  • virgl_hw_venus.h venus cap definitions

I would like to keep virgl_hw.h as the only header that other projects include directly. I am open as to which definitions go to which files, and how those files are named.

The new enum virgl_capsets remains in virgl_hw.h. I will see how #176 (closed) goes and moves/removes it accordingly.