vm: Fix broken hotunplugDisk

hotunplugDisk was broken by commit:
I4a19380c26ff4b6b697455e060dab4c651ff3f7c

Change-Id: I964ba4d117808254e9e9b4dd8a98d8c61457bf13
Signed-off-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17743
Reviewed-by: Dan Kenigsberg <danken@redhat.com>