[CORE] Introduce guest_irq_count attribute to specify maximum irqs of guest

This patch introduces guest_irq_count attribute in aspace node of guest instance. Using this attribute we specify maximum number of irqs in a guest instance.

In addition to guest interrupt controller, guest devices such as GPIO controller also use guest irq lines to emulate IO lines.

Total irqs of guests = Guest interrupt controller irqs + Dummy irqs used by non-PIC devices

This patch also renames h2g_irq_map attribute to host2guest_irq_map for readability.

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