crypto: introduce some common functions for af_alg backend

The AF_ALG socket family is the userspace interface for linux
crypto API, this patch adds af_alg family support and some common
functions for af_alg backend. It'll be used by afalg-backend crypto
latter.

Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>

Maintainer: modified to report an error if AF_ALG is requested
but cannot be supported

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

crypto/afalgpriv.h

@@ -0,0 +1,54 @@
+/*
+ * QEMU Crypto af_alg support
+ *
+ * Copyright (c) 2017 HUAWEI TECHNOLOGIES CO., LTD.
+ *
+ * Authors:
+ *    Longpeng(Mike) <longpeng2@huawei.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or
+ * (at your option) any later version.  See the COPYING file in the
+ * top-level directory.
+ */
+
+#ifndef QCRYPTO_AFALGPRIV_H
+#define QCRYPTO_AFALGPRIV_H
+
+#include <linux/if_alg.h>
+
+#define SALG_TYPE_LEN_MAX 14
+#define SALG_NAME_LEN_MAX 64
+
+typedef struct QCryptoAFAlg QCryptoAFAlg;
+
+struct QCryptoAFAlg {
+    int tfmfd;
+    int opfd;
+    struct msghdr *msg;
+    struct cmsghdr *cmsg;
+};
+
+/**
+ * qcrypto_afalg_comm_alloc:
+ * @type: the type of crypto operation
+ * @name: the name of crypto operation
+ *
+ * Allocate a QCryptoAFAlg object and bind itself to
+ * a AF_ALG socket.
+ *
+ * Returns:
+ *  a new QCryptoAFAlg object, or NULL in error.
+ */
+QCryptoAFAlg *
+qcrypto_afalg_comm_alloc(const char *type, const char *name,
+                         Error **errp);
+
+/**
+ * afalg_comm_free:
+ * @afalg: the QCryptoAFAlg object
+ *
+ * Free the @afalg.
+ */
+void qcrypto_afalg_comm_free(QCryptoAFAlg *afalg);
+
+#endif