Enable the QEMU guest agent interface in marionette VMs, run the
qemu-guest-agent service in one and try talking to it.
* gnu/build/marionette.scm (make-marionette): Enable the guest agent device.
* gnu/tests/virtualization.scm (run-qemu-guest-agent-test): New procedure.
(%test-qemu-guest-agent): New variable.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>