x86/IO-APIC: fix setup of Xen internally used IRQs (take 2)

x86/IO-APIC: fix setup of Xen internally used IRQs (take 2)

..., i.e. namely that of a PCI serial card with an IRQ above the
legacy range. This had got broken by the switch to cpumask_any() in
cpu_mask_to_apicid_phys(). Fix this by allowing all CPUs for that IRQ
(via setup_vector_irq() properly updating a booting CPU's vector_irq[],
thus avoiding "No irq handler for vector" messages and the interrupt
not working).

Cleanup coding style and types there at once.

While doing this I also noticed that io_apic_set_pci_routing() can't
be quite right: It sets up the destination _before_ getting a vector
allocated (which on other than systems using the flat APIC mode
affects the possible destinations), and also didn't restrict affinity
to ->arch.cpu_mask (as established by assign_irq_vector()).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>