vm: promote some debug messages to warn

Promote to 'warn' some logs which reveal
behaviour that may surprise the user.

This patch is part of the preparation for
the switch to default log level to 'info'
in VDSM.

Change-Id: Id7757e7215631f06e93014552363c2ed069ffa47
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34356
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>