vbox: Cleanup partially-defined VM on failure

vbox: Cleanup partially-defined VM on failure

Since the VBOX API requires to register an initial VM before proceeding
to attach any remaining devices to it, any failure to attach such
devices should result in automatic cleanup of the initially registered
VM so that the state of VBOX registry remains clean without any leftover
"aborted" VMs in it. Failure to cleanup of such partial VMs results in a
warning log so that actual define error stays on the top of the error
stack.