KVM: selftests: Update gitignore file for latest changes

The kvm_create_max_vcpus test has been moved to the main directory,
and sync_regs_test is now available on s390x, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>