powerpc: Refactor is_kvm_guest() declaration to new header

powerpc: Refactor is_kvm_guest() declaration to new header

[ Upstream commit 92cc6bf01c7f4c5cfefd1963985c0064687ebeda ]

Only code/declaration movement, in anticipation of doing a KVM-aware
vcpu_is_preempted(). No additional changes.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201202050456.164005-2-srikar@linux.vnet.ibm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>