backup: qemu: Add directory for tracking checkpoints

backup: qemu: Add directory for tracking checkpoints

This is similar to the existing directory for snapshots; the domain
will save one xml file per checkpoint, for reloading on the next
libvirtd restart.  Fortunately, since checkpoints mandate RNG
validation, we are assured that the checkpoint name will be usable as
a file name (no abuse of '../escape' as a checkpoint name, for
example).

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>