virt: sev-guest: Rename local guest message variables

Rename local guest message variables for more clarity.

No functional change.

Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240731150811.156771-3-nikunj@amd.com