x86/pci/xen: Use msi_for_each_desc()

Replace the about to vanish iterators.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20211206210748.198359105@linutronix.de