qemu*xml2*test: Invoke tests from a function

qemu*xml2*test: Invoke tests from a function

Refactor the code so that the test macros invoke a helper function with
no additional steps. This change prevents regressions in compilation
time when adding extra steps for the tests, which happen when the test
macro gets too complicated.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>