qemu_snapshot: fix detection if non-leaf snapshot isn't in active chain

qemu_snapshot: fix detection if non-leaf snapshot isn't in active chain

The condition was completely wrong. As per the comment for function
virDomainMomentIsAncestor() it checks that the first argument is
descendant of the second argument.

Consider the following snapshot tree for VM:

  s1
    |
    +- s2
    |   |
    |   +- s3
    |
    +- s4
        |
        +- s5 (current)

When deleting s2 with the original code we checked if
virDomainMomentIsAncestor(s2, s5) which would return false basically for
any snapshot as s5 is leaf snapshot so no children.

When deleting s2 with fixed code we check if
virDomainMomentIsAncestor(s5, s2) which still returns false but when
deleting s4 it will correctly return true.

Before this fix it fails with the following error:

    error: Failed to delete snapshot s2
    error: invalid argument: could not find base disk source in disk source chain

After the fix it fails with correct error:

    error: Failed to delete snapshot s2
    error: unsupported configuration: deletion of non-leaf external snapshot that is not in active chain is not supported

Resolves: https://issues.redhat.com/browse/RHEL-23212
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>