Add a new qemuMonitorJSONSetObjectProperty() method to support invocation
of the 'qom-set' JSON monitor command with a provided path, property, and
expected data type to set.

NOTE: The set API was added only for the purpose of the qemumonitorjsontest

The test code uses the same "/machine/i440fx" property as the get test and
attempts to set the "realized" property to "true" (which it should be set
at anyway).