[ARM] vgic: Fix SGI triggering for SMP guest

When VCPU A triggers SGI to VCPU B in SMP guest using VGIC distributor
registers, we need to wake up VCPU B from WFI because it may be waiting
for interrupt.

This patch gets HW-assited GIC emulation working for SMP guest.

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