x86/tdx: Prepare for using "INFO" call for a second purpose

The TDG.VP.INFO TDCALL provides the guest with various details about
the TDX system that the guest needs to run.  Only one field is currently
used: 'gpa_width' which tells the guest which PTE bits mark pages shared
or private.

A second field is now needed: the guest "TD attributes" to tell if
virtualization exceptions are configured in a way that can harm the guest.

Make the naming and calling convention more generic and discrete from the
mask-centric one.

Thanks to Sathya for the inspiration here, but there's no code, comments
or changelogs left from where he started.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Tested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: stable@vger.kernel.org