Live snapshot is not supported

[BubaVV]

In release 2.3.0, it is impossible to create snapshot of running VM. It is caused by cidata cdrom created as disk, generating such device XML:

<disk type="file" device="disk">
  <driver name="qemu" type="raw"/>
  <source file="/var/lib/virt-lightning/pool/vl_vm-cidata.qcow2" index="1"/>
  <backingStore/>
  <target dev="vdb" bus="virtio"/>
  <alias name="virtio-disk1"/>
  <address type="pci" domain="0x0000" bus="0x00" slot="0x07" function="0x0"/>
</disk>

Steps to reproduce:

1. Startup the simplest VM. Here is example of yaml:

   - name: vl_vm
   distro: ubuntu-20.04
   memory: 512
   root_disk_size: 5

2. Try to create snapshot of running VM: virsh snapshot-create-as --domain vl_vm --name snapshot_1

As a result, we have such error message: error: unsupported configuration: internal snapshot for disk vdb unsupported for storage type raw

Proposed PR #271 to fix an issue