KVM: selftests: Annotate guest ucall, printf, and assert helpers with __printf()

Annotate guest printf helpers with __printf() so that the compiler will
warn about incorrect formatting at compile time (see git log for how easy
it is to screw up with the formatting).
Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20231129224916.532431-5-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/include/test_util.h

@@ -191,7 +191,7 @@ static inline uint32_t atoi_non_negative(const char *name, const char *num_str)
 }
 
 int guest_vsnprintf(char *buf, int n, const char *fmt, va_list args);
-int guest_snprintf(char *buf, int n, const char *fmt, ...);
+__printf(3, 4) int guest_snprintf(char *buf, int n, const char *fmt, ...);
 
 char *strdup_printf(const char *fmt, ...) __attribute__((format(printf, 1, 2), nonnull(1)));
 

tools/testing/selftests/kvm/include/ucall_common.h

@@ -34,9 +34,10 @@ void ucall_arch_do_ucall(vm_vaddr_t uc);
 void *ucall_arch_get_ucall(struct kvm_vcpu *vcpu);
 
 void ucall(uint64_t cmd, int nargs, ...);
-void ucall_fmt(uint64_t cmd, const char *fmt, ...);
-void ucall_assert(uint64_t cmd, const char *exp, const char *file,
-		  unsigned int line, const char *fmt, ...);
+__printf(2, 3) void ucall_fmt(uint64_t cmd, const char *fmt, ...);
+__printf(5, 6) void ucall_assert(uint64_t cmd, const char *exp,
+				 const char *file, unsigned int line,
+				 const char *fmt, ...);
 uint64_t get_ucall(struct kvm_vcpu *vcpu, struct ucall *uc);
 void ucall_init(struct kvm_vm *vm, vm_paddr_t mmio_gpa);
 int ucall_nr_pages_required(uint64_t page_size);