perf: ignore LACT devices reporting zero VRAM (#753)

Ignore LACT devices that report zero total VRAM.

Some virtual GPUs on headless VMs report `MemTotalMB == 0` through LACT,
which makes them appear in performance monitoring despite not providing
useful memory data. Skip those entries so only usable GPU devices are
reported.

This makes performance monitoring cleaner on headless VMs with virtual
GPUs that report zero VRAM.

Co-authored-by: David Soušek <david.sousek@intelogy.co.uk>

internal/perf/monitor_unix.go

@@ -106,6 +106,9 @@ func tryLACT(ctx context.Context, every time.Duration, logger *logmon.Monitor) (
 					if err != nil {
 						continue
 					}
+					if stat.MemTotalMB == 0 {
+						continue
+					}
 					stats = append(stats, stat)
 				}
 				conn.Close()