virtio-pci: Added check for virtio device in PCI config cbs.

Now, if virtio device is not present on virtio-bus - pci config callbacks
will not lead to possible crush. The read will return "-1" which should be
interpreted by a driver that pci device may be unplugged.

Signed-off-by: Andrew Melnychenko <andrew@daynix.com>
Message-Id: <20210609095843.141378-3-andrew@daynix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio-pci.c

@@ -424,6 +424,11 @@ static uint64_t virtio_pci_config_read(void *opaque, hwaddr addr,
     VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
     uint32_t config = VIRTIO_PCI_CONFIG_SIZE(&proxy->pci_dev);
     uint64_t val = 0;
+
+    if (vdev == NULL) {
+        return UINT64_MAX;
+    }
+
     if (addr < config) {
         return virtio_ioport_read(proxy, addr);
     }
@@ -455,6 +460,11 @@ static void virtio_pci_config_write(void *opaque, hwaddr addr,
     VirtIOPCIProxy *proxy = opaque;
     uint32_t config = VIRTIO_PCI_CONFIG_SIZE(&proxy->pci_dev);
     VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
+
+    if (vdev == NULL) {
+        return;
+    }
+
     if (addr < config) {
         virtio_ioport_write(proxy, addr, val);
         return;