pmb.qemu.run: set serial to mon:stdio

This makes QEMU trap signals like Ctrl-C and send it to the guest
instead of terminating QEMU.

To quit QEMU with this option you can use [Ctrl-A] [x]
  or
[Ctrl-A] [c] and type 'quit' at the prompt.

pmb/qemu/run.py

@@ -118,7 +118,7 @@ def command_qemu(args, arch, img_path):
     command += ["-smp", str(os.cpu_count())]
     command += ["-m", str(args.memory)]
 
-    command += ["-serial", "stdio"]
+    command += ["-serial", "mon:stdio"]
     command += ["-drive", "file=" + img_path + ",format=raw,if=virtio"]
     if args.qemu_tablet:
         command += ["-device", "virtio-tablet-pci"]