qemu/virtio: fix reset with device removal

virtio pci registers its own reset handler, but fails to unregister it,
which will lead to crashes after device removal.  Solve this problem by
switching to qdev reset handler, which is automatically unregistered.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
