qga: correctly write to /sys/power/state on linux

Commit v9.0.0-343-g2048129625 introduced usage of
g_file_set_contents() function to write to /sys/power/state.
This function uses G_FILE_SET_CONTENTS_CONSISTENT flag to
g_file_set_contents_full(), which is implemented by creating
a temp file in the same directory and renaming it to the final
destination.  Which is not how sysfs works.

Here, there's not a big deal to do open/write/close - it becomes
almost the same as using g_file_set_contents[_full]().  But it
does not have surprises like this.

Also, since this is linux code, it should be ok to use %m in
the error reporting function.

Fixes: 2048129625 "qga/commands-posix: don't do fork()/exec() when suspending via sysfs"
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3057
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20250801115316.6845-1-mjt@tls.msk.ru>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

qga/commands-linux.c

@@ -1400,20 +1400,22 @@ static bool linux_sys_state_supports_mode(SuspendMode mode, Error **errp)
 
 static void linux_sys_state_suspend(SuspendMode mode, Error **errp)
 {
-    g_autoptr(GError) local_gerr = NULL;
     const char *sysfile_strs[3] = {"disk", "mem", NULL};
     const char *sysfile_str = sysfile_strs[mode];
+    int fd;
 
     if (!sysfile_str) {
         error_setg(errp, "unknown guest suspend mode");
         return;
     }
 
-    if (!g_file_set_contents(LINUX_SYS_STATE_FILE, sysfile_str,
-                             -1, &local_gerr)) {
-        error_setg(errp, "suspend: cannot write to '%s': %s",
-                   LINUX_SYS_STATE_FILE, local_gerr->message);
-        return;
+    fd = open(LINUX_SYS_STATE_FILE, O_WRONLY);
+    if (fd < 0 || write(fd, sysfile_str, strlen(sysfile_str)) < 0) {
+        error_setg(errp, "suspend: cannot write to '%s': %m",
+                   LINUX_SYS_STATE_FILE);
+    }
+    if (fd >= 0) {
+        close(fd);
     }
 }