libvirt: Don't pass None for image_meta parameter in tests

The get_guest_config() method in the libvirt driver expects to
have a non-None value for image_meta - the to_xml() caller
will ensure this is non-None. The test cases, however, are often
calling get_guest_config() directly instead of via to_xml() and
are passing None for image_meta. Fortunately this hasn't been a
problem historically but will become one as more of the XML code
requires access to the image_meta object. Ensure the test cases
all pass {} instead of None to addres this.

Blueprint: virt-driver-vcpu-topology
Change-Id: I30fc23ef1344fcdcfef10113c6679025f60f2036
Merge "libvirt: Don't pass None for image_meta parameter in tests"