virNetDevMacVLanTapSetup: Work around older systems

virNetDevMacVLanTapSetup: Work around older systems

Some older systems, e.g. RHEL-6 do not have IFF_MULTI_QUEUE flag
which we use to enable multiqueue feature. Therefore one gets the
following compile error there:

  CC     util/libvirt_util_la-virnetdevmacvlan.lo
util/virnetdevmacvlan.c: In function 'virNetDevMacVLanTapSetup':
util/virnetdevmacvlan.c:338: error: 'IFF_MULTI_QUEUE' undeclared (first use in this function)
util/virnetdevmacvlan.c:338: error: (Each undeclared identifier is reported only once
util/virnetdevmacvlan.c:338: error: for each function it appears in.)
make[3]: *** [util/libvirt_util_la-virnetdevmacvlan.lo] Error 1

So, whenever user wants us to enable the feature on such systems,
we will just throw a runtime error instead.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>