For keyboard is not totally imposibble to use it with USB qube, but it's
somehow harder to setup (and if something goes wrong, no keyboard will
be available). But currently if booted from USB disk, it is imposible to
work with USB qube then.
At the some time, slightly rework QubesChoice external conditions - have
them checked outside of the object, which make it easier to have
different message when such option is not available (and why).
FixesQubesOS/qubes-issues#1649FixesQubesOS/qubes-issues#1630