Implement profiles

[msm-code]

Currently when user runs:

vixos run firefox

This actually means many things:

• create a profile called "firefox", if it doesn't exist yet.
• download a package pkgs.firefox in it.
• and start an executable called firefox

It's possible to run a different executable in a firefox "profile" by specifying --executable flag. But it's not possible to install more packages there.

A better UI would be something like:

vixos run personal firefox

That would mean "download and run pkgs.firefox into a profile called personal". But users could also start bash, or file editor, etc.

Still thinking about the details, but I think profiles are the way to go (even QubesOS that we're stealing ideas from uses "profiles" for most of the work, and appvm are only a small part).