Add support for importing volume data with qvm-volume tool.
This could be also used to clear volume by issuing:
qvm-volume import --no-resize some-vm:private /dev/null
QubesOS/qubes-issues#5192
Right now Admin API backend will refuse to shrink volume anyway, but
we're planning to relax this restriction. Make sure the client side
(qvm-volume tool here, GUI VM settings already have this in place) will
employ appropriate safety check.
QubesOS/qubes-issues#3725
