Add option to uniformly start new DispVM from either VM or Dom0. This
use DispVMWrapper, which translate it to either qrexec call to $dispvm,
or (in dom0) to appropriate Admin API call to create fresh DispVM
first.
This require abandoning registering --all and --exclude by
QubesArgumentParser, because we need to add --dispvm mutually exclusive
with those two. But actually handling those two options is still done by
QubesArgumentParser.
This also updates man page and tests.
FixesQubesOS/qubes-issues#2974
37ae76823b