Handle instance directories correctly for migrates.

In Grizzly we changed how we name instance directories. Unfortunately
the decision as to what directory name to use is based on whether
there is a pre-existing directory on the compute node with the old
name.

In the case of live migration, that doesn't work because we have
libvirt copy the directory across for us. So, we need to pass
around the directory name in the migration request. This is better
anyway, as it means that ops types don't need to handle an instance
directory magically renaming itself at some point.

Resolves bug 1175286.

Change-Id: Ib102e644a8742726fa2049303354d466b8109e0b
Merge "Handle instance directories correctly for migrates."