ARM: virt: select ARM_GIC_V3

This patch allows ARM guests to use GICv3 on an arm64 host

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>