kvm tools: Make virt_queue__available return false if queue is not initialized

virtio_console__inject_interrupt tries to use virt queues before guest
tell us to initialize them.

(gdb) r run -i linux-0.2.img -k ./vmlinuz-2.6.38-rc6+ -r ./initrd.img-2.6.38-rc6+ -p=init=1 -m 500 -c
Starting program: /project/rh/kvm-tools/tools/kvm/kvm run -i linux-0.2.img -k ./vmlinuz-2.6.38-rc6+ -r ./initrd.img-2.6.38-rc6+ -p=init=1 -m 500 -c
[Thread debugging using libthread_db enabled]
[New Thread 0x7fffd6e2d700 (LWP 19280)]
  Warning: request type 8

Program received signal SIGSEGV, Segmentation fault.
0x00000000004026ca in virt_queue__available (vq=0x60d3c8) at include/kvm/virtio.h:31
31              return vq->vring.avail->idx !=  vq->last_avail_idx;
(gdb)
(gdb) bt
(gdb) p *vq
$2 = {vring = {num = 0, desc = 0x0, avail = 0x0, used = 0x0}, pfn = 0, last_avail_idx = 0}

include/kvm/virtio-console.h:
 59 void virtio_console__inject_interrupt(struct kvm *self)
....
 71         if (term_readable(CONSOLE_VIRTIO) && virt_queue__available(vq)) {
 72                 head = virt_queue__get_iov(vq, iov, &out, &in, self);
                           ^^^^ then this block will not be executed if
                                virtio_queue is unavaiable.

Changes from v1:
- move the check of virt_queue out of virt_queue__get_iov()

Reported-by: Amos Kong <akong@redhat.com>
Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

include/kvm/virtio.h

@@ -28,6 +28,8 @@ static inline struct vring_desc *virt_queue__get_desc(struct virt_queue *queue,
 
 static inline bool virt_queue__available(struct virt_queue *vq)
 {
+	if (!vq->vring.avail)
+		return 0;
 	return vq->vring.avail->idx !=  vq->last_avail_idx;
 }