x86/paravirt: return full 64-bit result

Impact: Bug fix

A hunk went missing in the original patch, and callee-save callsites were
not marked as returning the upper 32-bit of result, causing Badness.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>