[CORE] vmm_host_irq: Add a special flag for host-to-guest routed irqs

This patch adds a special per-host-irq flag to represent host-to-guest
routed irqs. The arch specific HW assisted PIC emulators (e.g. VGIC)
can use this per-host-irq flag to mark host irqs that are routed to
guest. The host PIC driver (e.g. GIC) can also check this flag to
handle host-to-guest routed irqs differently.

Signed-off-by: Anup Patel <anup@brainfault.org>