block: Catch attempt to attach multiple devices to a blockdev

For instance, -device scsi-disk,drive=foo -device scsi-disk,drive=foo
happily creates two SCSI disks connected to the same block device.
It's all downhill from there.

Device usb-storage deliberately attaches twice to the same blockdev,
which fails with the fix in place.  Detach before the second attach
there.

Also catch attempt to delete while a guest device model is attached.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/hw/pci-hotplug.c b/hw/pci-hotplug.c
index d743192..fe468d6 100644
--- a/hw/pci-hotplug.c
+++ b/hw/pci-hotplug.c
@@ -214,7 +214,11 @@
             return NULL;
         }
         dev = pci_create(bus, devfn, "virtio-blk-pci");
-        qdev_prop_set_drive(&dev->qdev, "drive", dinfo->bdrv);
+        if (qdev_prop_set_drive(&dev->qdev, "drive", dinfo->bdrv) < 0) {
+            qdev_free(&dev->qdev);
+            dev = NULL;
+            break;
+        }
         if (qdev_init(&dev->qdev) < 0)
             dev = NULL;
         break;