KVM: arm64: Add a capability to advertise SVE support

To provide a uniform way to check for KVM SVE support amongst other
features, this patch adds a suitable capability KVM_CAP_ARM_SVE,
and reports it as present when SVE is available.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Julien Thierry <julien.thierry@arm.com>
Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>