docs: kbase: Add a doc on live full disk backup

docs: kbase: Add a doc on live full disk backup

This is a rewrite of:

    https://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit

Once this commit merges, the above wiki should point to this kbase
document.

NB: I've intentionally left out the example for pull-based full backups.
I'll tackle it once QMP `x-blockdev-reopen` comes out of experimental
mode in upstream QEMU.  Then pull-based can be described for both full
and and differntial backups.

Overall, future documents should cover:

   - full backups using both push- and pull-mode
   - differential backups using both push- and pull-mode

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>