Bug 1813310

Description:
[incremental_backup] Cannot backup to a RAW file (even for full backup). But this is mentioned in libvirt doc.

Versions:
libvirt-6.0.0-9.module+el8.2.0+5957+7ae8988e.x86_64

How reproducible:
100%

Steps:
1. In libvirt-rhel/docs/formatbackup.html.in
Or https://libvirt.org/formatbackup.html#BackupAttributes
There is an example of backup's target driver type='raw'
===============================
target
Valid only for push mode backups, this is the primary sub-element that describes the file name of the backup destination, similar to the source sub-element of a domain disk. An optional sub-element driver can also be used, with an attribute type to specify a destination format different from qcow2.
...
...
<domainbackup>
  <disks>
    <disk name='vda' backup='yes'/>
    <disk name='vdb' type='file'>
      <target file='/path/to/vdb.backup'/>
      <driver type='raw'/>
    </disk>
    <disk name='vdc' backup='no'/>
  </disks>
</domainbackup>
===============================

2. Prepare a vm 
[root@hp-dl320eg8-05 push]# virsh domblklist vm1
 Target   Source
--------------------------------------------------
 vda      /var/lib/libvirt/images/base_os.active
 vdb      avocado-vt-pool/inc_test.qcow2

3. Prepare backup and checkpoint xml, and set target driver='raw'
[root@hp-dl320eg8-05 push]# cat backup_push_raw.xml
<domainbackup mode='push'>
  <disks>
    <disk name='vda' type='file'>
      <target file='/home/images/vda.full.backup'/>
      <driver type='raw'/>
    </disk>
    <disk name='vdb' backup='no'/>
  </disks>
</domainbackup>

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

4. Start the backup, failed.
[root@hp-dl320eg8-05 push]# virsh backup-begin vm1 backup_push_raw.xml checkpoint_push_full.xml
error: unsupported configuration: storage format 'raw' does not support backing store


Actual result:
Libvirt doc gave a sample with driver=raw, but actually it's not supported.

Expected result:
For full backup, maybe we should allow driver=raw, and forbid driver=raw for incremental backup?
If raw is forbidden due to we cannot use the full backup file as backing file for incremental backup later, we may need to update to doc to avoid a confusing example using driver=raw.