KVM: PPC: e500: clean up arch/powerpc/kvm/e500.h

Move vcpu to the beginning of vcpu_e500 to give it appropriate
prominence, especially if more fields end up getting added to the
end of vcpu_e500 (and vcpu ends up in the middle).

Remove gratuitous "extern" and add parameter names to prototypes.

Signed-off-by: Scott Wood <scottwood@freescale.com>
[agraf: fix bisectability]
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>