Avoid races between KVM_SET_GSI_ROUTING and KVM_CREATE_IRQCHIP by taking
the kvm->lock when setting up routes.

If KVM_CREATE_IRQCHIP fails, KVM_SET_GSI_ROUTING could have already set
up routes pointing at pic/ioapic, being silently removed already.

Also, as a side effect, this patch makes sure that KVM_SET_GSI_ROUTING
and KVM_CAP_SPLIT_IRQCHIP cannot run in parallel.
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>