KVM: Implement kvm_put_guest()

kvm_put_guest() is analogous to put_user() - it writes a single value to
the guest physical address. The implementation is built upon put_user()
and so it has the same single copy atomic properties.

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