x86/IRQ: eliminate irq_vector[]

x86/IRQ: eliminate irq_vector[]

The vector is already being tracked in struct irq_desc's arch.vector
member, so there's no real need for a second place where this to get
stored. The only caveat is that legacy vectors (used for interrupts
handled through the 8259) must be special cased to not prevent non-
legacy vectors from being assigned.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>