Catch InstanceNotFound exception if migration fails

When a VM with an attached iSCSI disk fails to migrate, the rollback methods
does not detach the disk from target host. What happens is _lookup_by_name()
fails, since the VM does not exist on the target host. In detach_volume(), it
is supposed to print a warning based on the correct error code being returned,
instead of throwing the exception. However, this is not happening, because
_lookup_by_name() throws an InstanceNotFound exception, rather than a
libvirt.libvirtError exception. So we also need to catch InstanceNotFound
exception, so that detach_volume() can continue to execute as expected.

Change-Id: I396c23d8137bf1e2834769c5483d2494506949ad
Closes-Bug: #1321082
Merge "Catch InstanceNotFound exception if migration fails"