Document caveats of hypervisor-specific stats in 'VIR_DOMAIN_STATS_VCPU' group

Document caveats of hypervisor-specific stats in 'VIR_DOMAIN_STATS_VCPU' group

In commit c43718ef67944 I've added a disclaimer that the new stats which
are fetched from qemu and passed directly to the user are not guaranteed
by libvirt. I didn't notice that per-vcpu hypervisor specific stats are
also snuck into the VIR_DOMAIN_STATS_VCPU group along with other
pre-existing stats we do guarantee.

Extend the disclaimer for VIR_DOMAIN_STATS_VCPU too.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>