It appears that you do not need to add virtualbox-kmp-default to the guest

Today’s update with the virtualbox-kmp-default resulted in black screen with no mouse or keyboard in the X11 session.
The console still worked so I did a zypper rm virtualbox-kmp-default and rebooted.
I still had virtualbox mouse and keyboard support - everything seems to work fine as a guest OS.

This is true for 15.2 as well.

Virtualbox 6.0.0r127566 seems to be built into the default image for Tumbleweed and 15.2.