This function can be called at any time to get the current status of a
guest's network device rx-filter. In particular it is useful to call
after libvirt recieves a NIC_RX_FILTER_CHANGED event - this event only
tells you that something has changed in the rx-filter, the details are
retrieved with the query-rx-filter monitor command (only available in
the json monitor). The command sent to the qemu monitor looks like this:

  {"execute":"query-rx-filter", "arguments": {"name":"net2"} }'

and the results will look something like this:

{
    "return": [
        {
            "promiscuous": false,
            "name": "net2",
            "main-mac": "52:54:00:98:2d:e3",
            "unicast": "normal",
            "vlan": "normal",
            "vlan-table": [
                42,
                0
            ],
            "unicast-table": [

            ],
            "multicast": "normal",
            "multicast-overflow": false,
            "unicast-overflow": false,
            "multicast-table": [
                "33:33:ff:98:2d:e3",
                "01:80:c2:00:00:21",
                "01:00:5e:00:00:fb",
                "33:33:ff:98:2d:e2",
                "01:00:5e:00:00:01",
                "33:33:00:00:00:01"
            ],
            "broadcast-allowed": false
        }
    ],
    "id": "libvirt-14"
}

This is all parsed from JSON into a virNetDevRxFilter object for
easier consumption. (unicast-table is usually empty, but is also an
array of mac addresses similar to multicast-table).

(NB: LIBNL_CFLAGS was added to tests/Makefile.am because virnetdev.h
now includes util/virnetlink.h, which includes netlink/msg.h when
appropriate. Without LIBNL_CFLAGS, gcc can't find that file (if
libnl/netlink isn't available, LIBNL_CFLAGS will be empty and
virnetlink.h won't try to include netlink/msg.h anyway).)