Add vmctl send and vmctl receive

As we see from the commit message, new developer Pratik Vyas (pd@) adds the ability to do paused VM migrations for VMM.

Mike Larkin also writes on Twitter:

#OpenBSD vmctl(8) : vmctl send myvm | ssh mlarkin@somewhere.com vmctl receive Yes, it's that easy.

Ed: this is *paused* migrations, not live migrations.