Update libvirt imagecache to support resizes.
Update libvirt imagecache to support resizes.

Updates the libvirt image cache manage so that it accounts
for instances which are resizing.

Also fixes an issue where an exception was thrown when
the image cache manager ran while instances were resizing.
This is because instances which are resizing may have files in the
Nova instances directory which are missing backing files.

Fixes LP Bug #1006616.

Change-Id: I724c6855b68eca24a00893fd96b26b1443f9da5c

n0ano/ganttclient