'dispvm_allowed' name was confusing, because it suggested being able to
spawn new DispVMs, not being a template for DispVM.
FixesQubesOS/qubes-issues#3047
Add public Admin API call to create Disposable VM that would be
automatically destroyed after shutdown. Do not keep this functionality
for qrexec-policy tool only.
Also, use admin.vm.Start there, instead of internal.vm.Start and
admin.vm.Kill instead of internal.vm.CleanupDispVM (this is enough,
because DispVM now have auto_cleanup property).
QubesOS/qubes-issues#2974
Implement this in two parts:
1. Permissions checks, getting a path from appropriate storage pool
2. Actual data import
The first part is done by qubesd in a standard way, but then, instead of
accepting all the data (which may be several GB), return a path to which
a shell script (in practice: `dd` command) will write the data.
Then the script call back to qubesd again to report success/failure and
qubesd response from that call is actually returned to the user.
This way we do not pass all the data through qubesd, but still can
control the process from there in a meaningful way. Note that the last
part (second call to qubesd) may perform all kind of verification (like
a signature check on the data, or so) and can also prevent VM from
starting (hooking also domain-pre-start event) from not verified image.
QubesOS/qubes-issues#2622
