In refactoring the snapshot code to prepare for checkpoints, I changed
qemuDomainMomentDiscardAll to take a callback that would handle the
cleanup of either a snapshot or a checkpoint, but failed to set the
callback on one of the two snapshot callers.  As a result, 'virsh
undefine $dom --snapshots-metadata' crashed on a NULL function
dereference.

Fixes: a487890d
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1707708Signed-off-by: NEric Blake <eblake@redhat.com>
Acked-by: NPeter Krempa <pkrempa@redhat.com>