Create shadow structs with named bitfields for msi_msg data, address_lo and
address_hi and use them in the MSI message composer.

Provide a function to retrieve the destination ID. This could be inline,
but that'd create a circular header dependency.

[dwmw2: fix bitfields not all to be a union]
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201024213535.443185-13-dwmw2@infradead.org