qemu: command: Refactor NUMA backend object formatting to use JSON objs

qemu: command: Refactor NUMA backend object formatting to use JSON objs

With the new JSON to argv formatter we are now able to represent the
memory backend definitions in the JSON object format that is reusable
for monitor use (hotplug) and then convert it into the shell string.
This will avoid having two separate instances of the same code that
would create the different formats.

Previous refactors now allow to make this step without changes to the
test suite.