This is for two reasons. First, validation can meet external mount
and will call plugins, which is not correct on pre-dump and actually
crashes on uninitilized plugins lists. Second, even if on pre-dump
mount tree is not "supported" this can be a temporary situation (yes,
yes, unlikely, but still).

On the other hand, it's better to fail earlier, but that's another
story.
Reported-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>