kvm: zero-initialize KVM_SET_GSI_ROUTING input

kvm_add_routing_entry makes an attempt to
zero-initialize any new routing entry.
However, it fails to initialize padding
within the u field of the structure
kvm_irq_routing_entry.

Other functions like kvm_irqchip_update_msi_route
also fail to initialize the padding field in
kvm_irq_routing_entry.

It's better to just make sure all input is initialized.

Once it is, we can also drop complex field by field assignment and just
do the simple *a = *b to update a route entry.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>