kvm: selftests: Support dirty log initial-all-set test

Since the new capability KVM_DIRTY_LOG_INITIALLY_SET of
KVM_CAP_MANUAL_DIRTY_LOG_PROTECT2 has been introduced, tweak the
clear_dirty_log_test to use it.

Signed-off-by: Jay Zhou <jianjay.zhou@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>