virtio: check virtio_load return code

Otherwise we crash on error.

Signed-off-by: Ulrich Obergfell <uobergfe@redhat.com>
Signed-off-by: Orit Wassermann <owasserm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
