iscsi: checking image size against VG free space

On block devices we are preallocating the disk image so we need to check
if we have enough free space before starting.

We need to validate the image size before creating the VG so we cannot simply
check the VG free space but we need a rough estimate of its overhead

Change-Id: I17f861a053f6ac38983967ffb07ecca9ff9b8de1
Bug-Url: https://bugzilla.redhat.com/1142710
Signed-off-by: Simone Tiraboschi <stirabos@redhat.com>