KVM: Allow kvm_device_ops to be const

Currently a kvm_device_ops structure cannot be const without triggering
compiler warnings. However the structure doesn't need to be written to
and, by marking it const, it can be read-only in memory. Add some more
const keywords to allow this.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>