KVM: selftests: Fix printf() format goof in SEV smoke test

Print out the index of mismatching XSAVE bytes using unsigned decimal
format.  Some versions of clang complain about trying to print an integer
as an unsigned char.

  x86/sev_smoke_test.c:55:51: error: format specifies type 'unsigned char'
                                     but the argument has type 'int' [-Werror,-Wformat]

Fixes: 8c53183d ("selftests: kvm: add test for transferring FPU state into VMSA")
Link: https://lore.kernel.org/r/20250228233852.3855676-1-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>