struct AddressSpaceChildDriverPingMessage

Child driver protocol

By default, creating connection to the driver does not associate any child

driver with the connection.

The client opens a child driver via OpenChildDriver, giving the type of the

driver as an argument along with a request for the connection. The type of

the driver is a number and the number/drivertype mapping is

determined/maintained in:

https://android.googlesource.com/platform/external/qemu/+/refs/heads/emu-master-dev/android/android-emu/android/emulation/AddressSpaceService.h

In Fuchsia, we currently only support the DEFAULT type, which is used for

graphics.

After opening the child driver, the client and hardware communicate via a

child driver-specific protocol, with notifications driven by `Ping()`, each

of which writes and reads messages to the hardware that follow this

`AddressSpaceChildDriverPingMessage` struct.

Each child driver type will have its own semantics for each field.

It's common for child drivers to refer to offset/size plus a metadata field.

We also provide extra data fields for other use cases in particular child

drivers.