x86/pci/xen: Use msi_msg shadow structs

Use the msi_msg shadow structs and compose the message with named bitfields
instead of the unreadable macro maze.

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/20201024213535.443185-18-dwmw2@infradead.org