virtio: correctly initialize vm_running

Current vm_running was not explicitly initialized and its value was changed by
vm state notifier, this may confuse the virtio device being hotplugged such as
virtio-net with vhost backend as it may think the vm was not running. Solve this
by initialize this value explicitly in virtio_common_init().

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
