samples: ipc: openamp_rsc_table: Send virtio ID as message content

The message data field is currently empty, this works for some as the all
available virtqueues can be signaled when any message comes in on the host
side. For TI Mailbox and RemoteProc Linux drivers we do check the content
and expect the message to contain the correct Vring ID to signal.

Signed-off-by: Andrew Davis <afd@ti.com>