irq_remapping: move structs outside #ifdef

This is friendlier to clients of the code, who are going to prepare
vcpu_data structs unconditionally, even if CONFIG_IRQ_REMAP is not
defined.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>