qemu driver: Support for Ceph block devices

QEMU has native support for [Ceph block devices](http://ceph.com/ceph-storage/block-storage/). Starting KVM with an RBD drive looks like this:

```
file=rbd:rbd/jessie:id=admin:key=xxx==:auth_supported=cephx\;none:mon_host=node1.example.com\:6789\;node2.example.com\:6789\;node3.example.com\:6789,if=none,id=drive-virtio-disk0,format=raw -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1
```

This could be supported by allowing to pass qemu custom args and making `artifact_source` optional.