Yes, that's one workaround. In my case, though, I needed to reset the root password in the VM. I intended to interrupt the boot sequence to start single-user mode.
I'll test some other VMs soon and see if I can reproduce the issue on all of them.
Using Proxmox VE 3.1, the console disconnects just after the VM displays "booting". So I must press Refresh and wait several seconds. This makes pressing Shift/Escape to interrupt GRUB impossible. Is there a workaround?