tests: qemuxml2xml: assign device addresses

tests: qemuxml2xml: assign device addresses

We use the PreFormat callback for this. Many test cases need to be extended
to pass in proper qemuCaps flags so AssignAddresses doesn't throw errors.

One test case (pcie-root-port-too-many) is dropped, since it was meant
only for checking an error condition in qemuxml2argv, and one we add in
AssignAddresses it errors here too.

Long term I think AssignAddresses should be handled in qemu's PostParse
callback, but that's not entirely straightforward. Handling it here
means we can get the test suite churn over with.