Allow detecting capabilities according to the qemu QMP schema. This is
necessary as sometimes the availability of certain options depends on
the presence of a field in the schema.

This patch adds support for loading the QMP schema when detecting qemu
capabilities and adds a very simple query language to allow traversing
the schema and selecting a certain element from it.

The infrastructure in this patch uses a query path to set a specific
capability flag according to the availability of the given element in
the schema.