qga-win: report specific error when failing to open channel

Useful in general, but especially now that errors might occur more
frequently with --retry-path set.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qga/channel-win32.c

@@ -302,7 +302,8 @@ static gboolean ga_channel_open(GAChannel *c, GAChannelMethod method,
                            OPEN_EXISTING,
                            FILE_FLAG_NO_BUFFERING | FILE_FLAG_OVERLAPPED, NULL);
     if (c->handle == INVALID_HANDLE_VALUE) {
-        g_critical("error opening path %s", newpath);
+        g_critical("error opening path %s: %s", newpath,
+                   g_win32_error_message(GetLastError()));
         return false;
     }