vzlist: don't spoil output with ploop messages

ploop library is pretty verbose, and some of the ploop functions
we call might end up printing something to stdout. This is especially
bad if -j is used, but even without it it's not good.

So let's use ploop_set_verbose_level() to fix it.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>