selftests/kvm: Replace attribute with macro

The __printf() macro is used in many tools in the linux kernel to
validate the format specifiers in functions that use printf. The kvm
selftest uses it without putting it in a macro definition while it
also imports the kselftests.h header where the macro attribute is
defined.

Use __printf() from kselftests.h instead of the full attribute.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>