virt: sev-guest: Fix user-visible strings

User-visible abbreviations should be in capitals, ensure messages are
readable and clear.

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/20240731150811.156771-4-nikunj@amd.com