qemu: Only set up base image device if requested
Adding a second virtual disk with the same UUIDs, partition labels, filesystem labels etc. can interfere with anything that mounts filesystems post-boot, and the attempts to address this in #842299 do not seem to have been completely successful. Nested virtualization is a rare thing to need, so let's make this opt-in.
Signed-off-by: Simon McVittie smcv@collabora.com Bug-Debian: https://bugs.debian.org/862362 Bug-Debian: https://bugs.debian.org/892023