virsh: Document limitation of domifstat

https://bugzilla.redhat.com/show_bug.cgi?id=1497396

The current implementation reads the stats from the host.
However, this doesn't work for all types of interfaces as not all
of them have a representation in the host. For instance,
interface type='user' doesn't.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

tools/virsh.pod

@@ -775,7 +775,11 @@ the guest OS via an agent. If unspecified, 'lease' is the default.
 
 =item B<domifstat> I<domain> I<interface-device>
 
-Get network interface stats for a running domain.
+Get network interface stats for a running domain. The network
+interface stats are only available for interfaces that have a
+physical source interface. This does not include, for example, a
+'user' interface type since it is a virtual LAN with NAT to the
+outside world.
 
 =item B<domif-setlink> I<domain> I<interface-device> I<state> [I<--config>]