KVM: powerpc: Change maintainer

Progress on KVM for Embedded PowerPC has stalled, but for Book3S there's quite
a lot of work to do and going on.

So in agreement with Hollis and Avi, we should switch maintainers for PowerPC.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Hollis Blanchard <hollis@penguinppc.org>
Signed-off-by: Avi Kivity <avi@redhat.com>