Implement file copy to/from VM

[msm-code]

It would be nice to be able to copy a file into and out of the VM. Planned syntax is:

$ vixos cp firefox:/path/in/vm ./relative/or/absolute/path

to copy from a VM

$ vixos cp ./relative/or/absolute/path firefox:/path/in/vm

to copy into a VM

$ vixos cp chrome:/path/in/vm firefox:/path/in/vm

to copy between VMs

$ vixos cp /path/a /path/b

to print "just use cp you dummy"

I don't think we need to support copying to/from a stopped VM, so we can implement it using SCP (we already have ssh server everywhere)