powerpc/kvm: Use correct CONFIG symbol in comment

This comment refers to the non-existent CONFIG_PPC_BOOK3S_XX, which
confuses scripts/checkkconfigsymbols.py.

Change it to use the correct symbol.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724131728.1643966-8-mpe@ellerman.id.au