KVM: arm64: Add a vcpu flag to control SVE visibility for the guest

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: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>