x86/xen: Wrap XEN MSI management into irqdomain

To allow utilizing the irq domain pointer in struct device it is necessary
to make XEN/MSI irq domain compatible.

While the right solution would be to truly convert XEN to irq domains, this
is an exercise which is not possible for mere mortals with limited XENology.

Provide a plain irqdomain wrapper around XEN. While this is blatant
violation of the irqdomain design, it's the only solution for a XEN igorant
person to make progress on the issue which triggered this change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20200826112333.622352798@linutronix.de