Since SVE will be enabled or disabled on a per-vcpu basis, a flag
is needed in order to track which vcpus have it enabled.

This patch adds a suitable flag and a helper for checking it.
Signed-off-by: NDave Martin <Dave.Martin@arm.com>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Tested-by: Nzhang.lei <zhang.lei@jp.fujitsu.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>