tests: Add testing of qemu migration cookie

tests: Add testing of qemu migration cookie

Migration cookie transports a lot of information but there are no tests
for it.

The test supports both xml2xml testing and also testing of the
population of the migration cookie data from a domain object, although
that option is not very useful as many things are collected from running
qemu and thus can't be tested efficiently here.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>