Fix fake libvirt XML generation for disks
Fix fake libvirt XML generation for disks

The fake libvirt driver generates disk xml for disks of various types,
but assumes file type for the source attribute. This causes a parse
failure for block devices.

No tests currently rely on this. I encountered it whilst writing a test
which did use a fake block volume, and it was a severe headscratcher
when the resulting parsed configuration had an empty source_path: being
a block device it was looking for source/dev, not source/file. I
ultimately didn't use that test, but would like to save somebody the
hairloss in the future.

Change-Id: I5cf594cb0cc8310ccfa4f31d981803ce152a69d5

openstack/nova,gooddata/openstack-nova,gooddata/openstack-nova,mahak/nova,mikalstill/nova,gooddata/openstack-nova,openstack/nova,klmitch/nova,rahulunair/nova,gooddata/openstack-nova,klmitch/nova,klmitch/nova,mahak/nova,rahulunair/nova,mikalstill/nova,klmitch/nova,openstack/nova,rahulunair/nova,phenoxim/nova,phenoxim/nova,mahak/nova,mikalstill/nova