vm: make the 'destroyed' field private

it is used only internally, so no reason to expose it.

Change-Id: I62d1296bf9f2e393b345d7b6efcc60d094144c8c
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31355
Reviewed-by: Dan Kenigsberg <danken@redhat.com>