tests: qemu: Add infrastructure for QAPI schema testing
    
    Add a function which will allow to test whether a JSON object conforms
    to the QAPI schema. This greatly helps when developing formatters for
    new JSON objects and will help make sure that the code will not break in
    cases which have unit tests but were actually not function-tested
    (mostly various disk access protocols).
    
    Signed-off-by: Peter Krempa <pkrempa@redhat.com>