Allow deleting running VMs

[mcandre]

As a user who frequently spins up and tears down many VMs, the current GUI restriction on deleting only non-running VMs is annoying.

Please enable the Remove option when VMs are running, so that users can efficiently delete VMs. It's one thing to show a confirmation prompt on deletion, another to hide the option altogether :/

[Socratis]

So... closing a VM and deleting it, is one step too many for you?

And it's not a GUI restriction either. Trying to unregister and delete a VM results in an error as well:

$ VBoxManage unregistervm "VMname" --delete
VBoxManage: error: Cannot unregister the machine 'VMname' while it is locked
VBoxManage: error: Details: code VBOX_E_INVALID_OBJECT_STATE (0x80bb0007), component MachineWrap, interface IMachine, callee nsISupports
VBoxManage: error: Context: "Unregister(CleanupMode_DetachAllReturnHardDisksOnly, ComSafeArrayAsOutParam(aMedia))" at line 154 of file VBoxManageMisc.cpp