Don't pass empty image to filter on live migration

If live migration for a volume backed instance without an image supplied
is initiated with no destination supplied, glance client show will not
be called and the image variable will be set to None.

However if passed to filters like that as part of the request_spec
dictionary, None value will cause one of the default filters - the
ImagePropertiesFilter to throw an exception in it's host_passes method,
as it expects an image to be a dictionary.

We solve this by leaving out the image entirely before engaging filters,
and not attempting to get it if it was not supplied to the instance.

This bug was previously fixed (however not fully, as the None image
would still cause an exception) in the following patch:
Ie683a3ca5d6430c52ead77b41f98fbcb4114ea1e, but  was accidentally dropped
in I6de33ada6dc377e20f8df07da92244f2c150b9fe.

Fixes bug 1170596 (again)
Fixes bug 1199811

Change-Id: I0239b6ac5640d075f937f9690eca4ce54f534117
Merge "Don't pass empty image to filter on live migration"