win32: add readv/writev emulation

Commit e9d8fbf (qemu-file: do not use stdio for qemu_fdopen, 2013-03-27)
introduced a usage of writev, which mingw32 does not have.  Even though
qemu_fdopen itself is not used on mingw32, the future-proof solution is
to add an implementation of it.  This is simple and similar to how we
emulate sendmsg/recvmsg in util/iov.c.

Some files include osdep.h without qemu-common.h, so move the definition
of iovec to osdep.h too, and include osdep.h from qemu-common.h
unconditionally (protection against including files when NEED_CPU_H is
defined is not needed since the removal of AREG0).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/osdep.h

@@ -1,6 +1,7 @@
 #ifndef QEMU_OSDEP_H
 #define QEMU_OSDEP_H
 
+#include "config-host.h"
 #include <stdarg.h>
 #include <stddef.h>
 #include <stdbool.h>
@@ -161,6 +162,22 @@ int qemu_close(int fd);
 int qemu_create_pidfile(const char *filename);
 int qemu_get_thread_id(void);
 
+#ifndef CONFIG_IOVEC
+struct iovec {
+    void *iov_base;
+    size_t iov_len;
+};
+/*
+ * Use the same value as Linux for now.
+ */
+#define IOV_MAX 1024
+
+ssize_t readv(int fd, const struct iovec *iov, int iov_cnt);
+ssize_t writev(int fd, const struct iovec *iov, int iov_cnt);
+#else
+#include <sys/uio.h>
+#endif
+
 #ifdef _WIN32
 static inline void qemu_timersub(const struct timeval *val1,
                                  const struct timeval *val2,