KVM: selftests: Convert x86's set BSP ID test to printf style guest asserts

Convert the set_boot_cpu_id test to use printf-based guest asserts,
specifically the EQ and NE variants.

Link: https://lore.kernel.org/r/20230729003643.1053367-27-seanjc@google.com


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