qmi-message: add functions to operate on QMI part of QmiMessage

QRTR-based transports will not have QMUX headers, and will instead encode that information in a different way. However, that data will still need to be passed around as part of the message by the library consumer, so reusing the QMUX header structure to pass it around should be fine and we will need to be able to create fake QMUX headers.

Also add a unit test for the new functions.