iov: add some functions to manage iovec

Introduce functions to copy to/from a buffer from/to an iovec array,
to compute data length in in bytes of an iovec and to copy memory from
an iovec to another.

iov_from_buf(), iov_to_buf(), iov_size(), iov_copy().

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-ID: <20240217150725.661467-2-lvivier@redhat.com>
[dwg: Small changes to suppress cppcheck warnings]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

iov.h

@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * iov.c - helpers for using (partial) iovecs.
+ *
+ * Copyrigh Red Hat
+ * Author: Laurent Vivier <lvivier@redhat.com>
+ *
+ * This file also contains code originally from QEMU include/qemu/iov.h:
+ *
+ * Author(s):
+ *  Amit Shah <amit.shah@redhat.com>
+ *  Michael Tokarev <mjt@tls.msk.ru>
+ */
+
+#ifndef IOVEC_H
+#define IOVEC_H
+
+#include <unistd.h>
+#include <string.h>
+
+size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
+                    size_t offset, const void *buf, size_t bytes);
+size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,
+                  size_t offset, void *buf, size_t bytes);
+size_t iov_size(const struct iovec *iov, size_t iov_cnt);
+unsigned iov_copy(struct iovec *dst_iov, size_t dst_iov_cnt,
+		  const struct iovec *iov, size_t iov_cnt,
+		  size_t offset, size_t bytes);
+#endif /* IOVEC_H */