rollback: correctly evaluate the space on the VM disk
rollback: correctly evaluate the space on the VM disk

Evaluate the real maximum space on the VM disk using the capacity
attribute instead of the apparentsize one.

Change-Id: Ie33d7d29790011077e485904949917a04763558c
Bug-Url: https://bugzilla.redhat.com/1397810
Signed-off-by: Simone Tiraboschi <cb03129c66ba4da7ba7044049d0f43907bf857bb@redhat.com>

oVirt/ovirt-hosted-engine-setup,oVirt/ovirt-hosted-engine-setup