release/Makefile.vm: Rework emulator-portinstall

The emulator-portinstall target now unconditionally ensures that qemu
is installed; but is only invoked if needed (aka. when cross building
VM images).

MFC After: 3 days
MFC With: 97bd53ef4d20 ("Fix duplicate rc.conf files")

(cherry picked from commit 7d0ee5ebd052d35e323f2d303e467c2cf305ca39)