Description of problem:
[incremental_backup] RFE: support luks encryption when do backup

Version-Release number of selected component (if applicable):
libvirt-6.0.0-9.module+el8.2.0+5957+7ae8988e.x86_64

How reproducible:
100%

Steps to Reproduce:
1. a vm has vdb (here is a luks image, but vdb doesn't require to be luks)
[root@hp-dl320eg8-05 push]# virsh dumpxml vm1 | awk '/<disk/,/<\/disk/'
...
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/vdb_luks.qcow2' index='1'/>
      <backingStore/>
      <target dev='vdb' bus='virtio'/>
      <encryption format='luks'>
        <secret type='passphrase' uuid='f981dd17-143f-45bc-88e6-222222222222'/>
      </encryption>
      <alias name='virtio-disk1'/>
      <address type='pci' domain='0x0000' bus='0x07' slot='0x00' function='0x0'/>
    </disk>

2. prepare backup and checkpoint xml
[root@hp-dl320eg8-05 push]# cat backup_push_full_luks.xml
<domainbackup mode='push'>
  <disks>
    <disk name='vdb' type='file'>
      <target file='/home/images/vdb.full.backup'/>
      <driver type='qcow2'/>
      <encryption format='luks'>
          <secret type='passphrase' uuid='f981dd17-143f-45bc-88e6-222222222222'/>
      </encryption>
    </disk>
  </disks>
</domainbackup>

 [root@hp-dl320eg8-05 push]# cat checkpoint_push_full.xml
<domaincheckpoint>
  <name>check_full</name>
</domaincheckpoint>

3. do the full backup
[root@hp-dl320eg8-05 push]# virsh backup-begin vm1 backup_push_full.xml checkpoint_push_full.xml
Backup started

4. the backup file is exported as a qcow2 file, encryption not supported
[root@hp-dl320eg8-05 push]# qemu-img info /home/images/vdb.full.backup
image: /home/images/vdb.full.backup
file format: qcow2
virtual size: 1 GiB (1073741824 bytes)
disk size: 2 GiB
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

Actual results:
cannot export a luks encrypted backup file

Expected results:
can export a luks encrypted backup file

Additional info:
If this is not supported, sometimes a luks encrypted disk will be backuped as a plaintext disk. 
blockcopy can support luks as follow:
[root@hp-dl320eg8-05 inc_bkup]# virsh dumpxml vm1 | awk '/<disk/,/<\/disk/'
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/base_os.active' index='1'/>
      <backingStore/>
      <target dev='vda' bus='virtio'/>
      <alias name='virtio-disk0'/>
      <address type='pci' domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
    </disk>


[root@hp-dl320eg8-05 inc_bkup]# cat ~/test.xml
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/tmp/blockcopy'>
        <encryption format='luks'>
          <secret type='passphrase' uuid='f981dd17-143f-45bc-88e6-222222222222'/>
        </encryption>
      </source>
    </disk>


[root@hp-dl320eg8-05 inc_bkup]# virsh blockcopy vm1 vda --xml ~/test.xml --transient-job --pivot --verbose
Block Copy: [100 %]
Successfully pivoted


[root@hp-dl320eg8-05 inc_bkup]# virsh dumpxml vm1 | awk '/<disk/,/<\/disk/'
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/tmp/blockcopy' index='2'>
        <encryption format='luks'>
          <secret type='passphrase' uuid='f981dd17-143f-45bc-88e6-222222222222'/>
        </encryption>
      </source>
      <backingStore/>
      <target dev='vda' bus='virtio'/>
      <alias name='virtio-disk0'/>
      <address type='pci' domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
    </disk>

[root@hp-dl320eg8-05 inc_bkup]# qemu-img info /tmp/blockcopy  -U
image: /tmp/blockcopy
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 75.5 MiB
encrypted: yes