backingchain:add case for blockcopy wth different dest xml

[nanli1]

VIRT-294410: Do blockcopy with different dest XML Signed-off-by: nanli [email protected]

 /usr/local/bin/avocado run --vt-type libvirt --test-runner=runner --vt-machine-type q35 backingchain.blockcopy.dest_xml
 (1/8) type_specific.io-github-autotest-libvirt.backingchain.blockcopy.dest_xml.file_disk.finish_reuse_external: PASS (45.10 s)
 (2/8) type_specific.io-github-autotest-libvirt.backingchain.blockcopy.dest_xml.file_disk.pivot_reuse_external: PASS (46.90 s)
 (3/8) type_specific.io-github-autotest-libvirt.backingchain.blockcopy.dest_xml.block_disk.finish_reuse_external: PASS (63.41 s)
 (4/8) type_specific.io-github-autotest-libvirt.backingchain.blockcopy.dest_xml.block_disk.pivot_reuse_external: -ujPASS (64.71 s)
 (5/8) type_specific.io-github-autotest-libvirt.backingchain.blockcopy.dest_xml.rbd_with_auth_disk.finish_reuse_external: PASS (48.02 s)
 (6/8) type_specific.io-github-autotest-libvirt.backingchain.blockcopy.dest_xml.rbd_with_auth_disk.pivot_reuse_external: PASS (46.32 s)
 (7/8) type_specific.io-github-autotest-libvirt.backingchain.blockcopy.dest_xml.nbd_disk.finish_reuse_external: PASS (49.41 s)
 (8/8) type_specific.io-github-autotest-libvirt.backingchain.blockcopy.dest_xml.nbd_disk.pivot_reuse_external: PASS (49.33 s)