govmm uses deprecated 'props' field on 'object-add' commands

[dgibson]

Description of problem

For both memory and NVDIMM adds, govmm needs to add memory-backend-file objects using the object-add qmp command.

Currently, govmm puts various properties for the new object in the props field of the args field to the basic QMP request. However use of props has been deprecated in qemu since 5.0 (commit 5f07c4d60d09) and is removed entirely in qemu 6.0 (commit 50243407457a).

Expected result

govmm uses modern QEMU QMP syntax so it works with current qemu versions.