Makefile.vm: Set CLEANDIRS correctly for vm-image

With multiple filesystems and disk image formats, we now use different
staging directories; we need to include them all in CLEANDIRS.

Fixes: 97bd53ef4d20b "Makefile.vm: Fix duplicate rc.conf files"
MFC after: 3 days

(cherry picked from commit 7ffa1f14e3c19f0061ec9f38bb74c564a49ffc17)