Add test for virt-resize.

Add test for virt-resize.

This tests a number of things which have caused problems for us:

 - resizing PVs and LV content
 - handling GPT format disks
 - using qcow2 as a target disk format
 - shrinking disk images

Note that the disk content is empty (not a real VM), but this is
adequate since all we want to test are the operations and calculations
done by virt-resize.  We are not interested here in whether e2fsprogs
and LVM actually works.