powerpc/kvm: Explicitly mark kvm guest code as __init

All the code in kvm.c can be marked __init. Most of it is already
inlined into the initcall, but not all. So instead of relying on the
inlining, mark it all as __init. This saves ~280 bytes of text for my
configuration.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190911115746.12433-3-mpe@ellerman.id.au