qga: Improve Windows filesystem space info retrieval logic

Previously, disk space reporting only worked for volumes with drive letters,
skipping those without (e.g. System Reserved).

This change always calls GetDiskFreeSpaceEx with fs->name, which is a
volume GUID path. Windows APIs accept both drive letters (e.g. "C:\")
and volume GUIDs (e.g. "\\?\Volume{GUID}\") as valid lpDirectoryName
parameters, so space reporting is now consistent across all volumes.

Reference:
https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file

Signed-off-by: minglei.liu <minglei.liu@smartx.com>
Reviewed-by: Kostiantyn Kostiuk <kkostiuk@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250923113243.78244-1-minglei.liu@smartx.com
Signed-off-by: Kostiantyn Kostiuk <kkostiuk@redhat.com>

qga/commands-win32.c

@@ -1164,15 +1164,15 @@ static GuestFilesystemInfo *build_guest_fsinfo(char *guid, Error **errp)
         fs->mountpoint = g_strdup("System Reserved");
     } else {
         fs->mountpoint = g_strndup(mnt_point, len);
-        if (GetDiskFreeSpaceEx(fs->mountpoint,
-                               (PULARGE_INTEGER) & i64FreeBytesToCaller,
-                               (PULARGE_INTEGER) & i64TotalBytes,
-                               (PULARGE_INTEGER) & i64FreeBytes)) {
-            fs->used_bytes = i64TotalBytes - i64FreeBytes;
-            fs->total_bytes = i64TotalBytes;
-            fs->has_total_bytes = true;
-            fs->has_used_bytes = true;
-        }
+    }
+    if (GetDiskFreeSpaceEx(fs->name,
+                            (PULARGE_INTEGER) & i64FreeBytesToCaller,
+                            (PULARGE_INTEGER) & i64TotalBytes,
+                            (PULARGE_INTEGER) & i64FreeBytes)) {
+        fs->used_bytes = i64TotalBytes - i64FreeBytes;
+        fs->total_bytes = i64TotalBytes;
+        fs->has_total_bytes = true;
+        fs->has_used_bytes = true;
     }
     wcstombs(fs_name, wfs_name, sizeof(wfs_name));
     fs->type = g_strdup(fs_name);