Package anon-shared-desktop is installed on all platforms.
There is no hypervisor detection in the build script. Not in that sense.
FYI: Redistributable KVM and VirtualBox images are created in one run.
--target qcow2 --target virtualbox
Not two. Non-ideal, but in the absence of more help, the only way not causing too much maintenance overhead.
Build switch --target qcow2 sets variable WHONIX_BUILD_QCOW2 to true and --target virtualbox sets variable WHONIX_BUILD_VIRTUALBOX to true. These variables influence Whonix's build script.
Example, the spice-vdagent is only installed when using --target qcow2, so builds using only --target root or --target qubes are spared from it.