nwfilter: change methods returning virNWFilterIPAddrLearnReq to use bool
Various methods return a virNWFilterIPAddrLearnReq struct, but the
callers are only interested in whether the return value is non-NULL.
It is thus preferrable to just return a bool.
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>