util: query/set BR_ISOLATED flag on netdevs attached to bridge

util: query/set BR_ISOLATED flag on netdevs attached to bridge

When this flag is set for an interface attached to a bridge, traffic
to/from the specified interface can only enter/exit the bridge via
another attached interface that *doesn't* have the BR_ISOLATED flag
set. This can be used to permit guests to communicate with the rest of
the network, but not with each other.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>