libvirt: Skip intermediate base files with qcow2.
libvirt: Skip intermediate base files with qcow2.

Instead of having two files in _base (the original and a resized
copy), let's just keep the originals and resize with the qcow2
image in the instance's directory. This will reduce the size of
_base and simplify cleanup. This also simplifies resizing of
instance disk images later.

Resolves bug 1087031.

Change-Id: Id91426e3cb9f75f31339b5156785e3782a4cb98f

n0ano/ganttclient