qapi: report the default CPU type for each machine

When user doesn't request any explicit CPU model with libvirt or QEMU,
a machine type specific CPU model is picked. Currently there is no way
to determine what this QEMU built-in default is, so libvirt cannot
report this back to the user in the XML config.

This extends the "query-machines" QMP command so that it reports the
default CPU model typename for each machine.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190822100412.23746-1-berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>