Run vhalt in parallel when used in lhalt

[AdamBromiley]

Is your feature request related to a problem? Please describe.
lhalt is notoriously slow because it runs each underlying vhalt command in sequence, which are each unavoidably slow already.

Describe the solution you'd like
Like how lstart can run multiple vstart commands in parallel (controllable with lab.dep, -p, and -s/--sequential), lhalt should run vhalt commands in parallel with similar CLI options.

This should be relatively simple to implement; just run each vhalt command in the background. The stdout of vhalt will need to be changed or hidden by lhalt, since it requires uninterrupted output for many seconds.