powerpc: move hmi.c to arch/powerpc/kvm/

hmi.c functions are unused unless sibling_subcore_state is nonzero, and
that in turn happens only if KVM is in use.  So move the code to
arch/powerpc/kvm/, putting it under CONFIG_KVM_BOOK3S_HV_POSSIBLE
rather than CONFIG_PPC_BOOK3S_64.  The sibling_subcore_state is also
included in struct paca_struct only if KVM is supported by the kernel.

Cc: Daniel Axtens <dja@axtens.net>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: kvm-ppc@vger.kernel.org
Cc: kvm@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>