net: add support of mac-programming over macvtap in QEMU side

Currently macvtap based macvlan device is working in promiscuous
mode, we want to implement mac-programming over macvtap through
Libvirt for better performance.

Design:
 QEMU notifies Libvirt when rx-filter config is changed in guest,
 then Libvirt query the rx-filter information by a monitor command,
 and sync the change to macvtap device. Related rx-filter config
 of the nic contains main mac, rx-mode items and vlan table.

This patch adds a QMP event to notify management of rx-filter change,
and adds a monitor command for management to query rx-filter
information.

Test:
 If we repeatedly add/remove vlan, and change macaddr of vlan
 interfaces in guest by a loop script.

Result:
 The events will flood the QMP client(management), management takes
 too much resource to process the events.

 Event_throttle API (set rate to 1 ms) can avoid the events to flood
 QMP client, but it could cause an unexpected delay (~1ms), guests
 guests normally expect rx-filter updates immediately.

 So we use a flag for each nic to avoid events flooding, the event
 is emitted once until the query command is executed. The flag
 implementation could not introduce unexpected delay.

There maybe exist an uncontrollable delay if we let Libvirt do the
real change, guests normally expect rx-filter updates immediately.
But it's another separate issue, we can investigate it when the
work in Libvirt side is done.

Michael S. Tsirkin: tweaked to enable events on start
Michael S. Tsirkin: fixed not to crash when no id
Michael S. Tsirkin: fold in patch:
   "additional fixes for mac-programming feature"
Amos Kong: always notify QMP client if mactable is changed
Amos Kong: return NULL list if no net client supports rx-filter query

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
