Define what the confidential VMBus is and describe what advantages it offers on the capable hardware. Signed-off-by: Roman Kisel <romank@linux.microsoft.com> Reviewed-by: Alok Tiwari <alok.a.tiwari@oracle.com> Reviewed-by: Michael Kelley <mhklinux@outlook.com> Signed-off-by: Wei Liu <wei.liu@kernel.org>