tests: qemublock: Add testing of blockdev JSON property generator

tests: qemublock: Add testing of blockdev JSON property generator

Add a test infrastructure that will allow testing the JSON object
generator used for generating data to use with blockdev-add.

The resulting disk including the backing chain is validated to conform
to the QAPI schema and the expected output files.

The first test cases make sure that libvirt will not allow nodenames
exceeding 31 chars.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>