Description of problem:
[incremental_backup][pull_mode] nbd exprotname and exportbitmap should be able to be set respectively

Version-Release number of selected component (if applicable):
libvirt-6.0.0-17.module+el8.2.0+6257+0d066c28.x86_64

How reproducible:
100%

Steps to Reproduce:
1. prepare backup xml and checkpoint xml. Pls note, the disk tag for vdb in the backup.xml having the optional "exportname" set as "vdb_custom_exp" 
(.libvirt-ci-venv-ci-runtest-l95mHu) [root@dell-per740xd-11 inc_bkup]# cat backup.xml
<domainbackup mode="pull">
    <server name="localhost" port="10809"/>
    <disks>
        <disk backup="no" name="vda"/>
        <disk backup="yes" exportname="vdb_custom_exp" name="vdb" type="file">
            <scratch file="/var/tmp/scratch_file_0" />
        </disk>
    </disks>
</domainbackup>

(.libvirt-ci-venv-ci-runtest-l95mHu) [root@dell-per740xd-11 inc_bkup]# cat checkpoint.xml
<domaincheckpoint>
  <name>check_full</name>
  <disks>
    <disk name='vda' checkpoint='no'/>
    <disk name='vdb' checkpoint='bitmap'/>
  </disks>
</domaincheckpoint>

2. start the backup, backup.xml hits rng validation failure 
(.libvirt-ci-venv-ci-runtest-l95mHu) [root@dell-per740xd-11 inc_bkup]# virsh backup-begin backup-test backup.xml checkpoint.xml
error: XML document failed to validate against schema: Unable to validate doc against /usr/share/libvirt/schemas/domainbackup.rng
Extra element server in interleave
Element domainbackup failed to validate content


3. let's have a check about the domainbackup.rng
(.libvirt-ci-venv-ci-runtest-l95mHu) [root@dell-per740xd-11 inc_bkup]# cat /usr/share/libvirt/schemas/domainbackup.rng | egrep "exportname|exportbitmap" -a4
...
            <optional>
              <attribute name='exportname'>
                <text/>
              </attribute>
              <attribute name='exportbitmap'>
                <text/>
              </attribute>
            </optional>
...
<==== in above xml, we can see the exportname and exportbitmap are put together in a pair of <optional/>. So they have to show up together, or a failure will happen. This is not reasonable, user should be able to customize nbd export name or bitmap name respectively. 


Actual results:
As above

Expected results:
The rng file should be something like:
            <optional>
              <attribute name='exportname'>
                <text/>
              </attribute>
+++         </optional>
+++         <optional>
              <attribute name='exportbitmap'>
                <text/>
              </attribute>
            </optional>