kvm: error out of kvm_irqchip_add_msi_route() in case of full route table

kvm: error out of kvm_irqchip_add_msi_route() in case of full route table

subj is calling kvm_add_routing_entry() which simply extends
  KVMState::irq_routes::entries[]
but doesn't check if number of routes goes beyond limit the kernel
is willing to accept. Which later leads toi the assert

  qemu-kvm: ../accel/kvm/kvm-all.c:1833: kvm_irqchip_commit_routes: Assertion `ret == 0' failed

typically it happens during guest boot for large enough guest

Reproduced with:
  ./qemu --enable-kvm -m 8G -smp 64 -machine pc \
     `for b in {1..2}; do echo -n "-device pci-bridge,id=pci$b,chassis_nr=$b ";
        for i in {0..31}; do touch /tmp/vblk$b$i;
           echo -n "-drive file=/tmp/vblk$b$i,if=none,id=drive$b$i,format=raw
                    -device virtio-blk-pci,drive=drive$b$i,bus=pci$b ";
      done; done`

While crash at boot time is bad, the same might happen at hotplug time
which is unacceptable.
So instead calling kvm_add_routing_entry() unconditionally, check first
that number of routes won't exceed KVM_CAP_IRQ_ROUTING. This way virtio
device insteads killin qemu, will gracefully fail to initialize device
as expected with following warnings on console:
    virtio-blk failed to set guest notifier (-28), ensure -accel kvm is set.
    virtio_bus_start_ioeventfd: failed. Fallback to userspace (slower).

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-ID: <20240408110956.451558-1-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>