KVM: selftests: Print the errno besides error-string in TEST_ASSERT

Print the errno besides error-string in TEST_ASSERT in the format of
"errno=%d - %s" will explicitly indicate that the string is an error
information. Besides, the errno is easier to be used for debugging
than the error-string.

Suggested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20210330080856.14940-5-wangyanan55@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>