vm: catch getVolumeSize failures in _normalizeVdsmImg

When getVolumeSize fails during the VM startup (e.g. volume is missing
or corrupted) a descriptive error should be returned (instead of the
previous KeyError: 'truesize'). An example of this behavior can be found
in BZ#883858.

Change-Id: Ib8e170a85100e57d7e644614a21038d869777b82
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13529
Reviewed-by: Ayal Baron <abaron@redhat.com>