net: introduce MAC_TABLE_CHANGED event

Introduce this new QMP event to notify management after guest changes
mac-table configuration.

Signed-off-by: Amos Kong <akong@redhat.com>
net: introduce command to query mac-table information

We want to implement mac programming over macvtap through Libvirt.
The previous patch adds QMP event to notify management of mac-table
change. This patch adds a monitor command to query rx mode information
of mac-tables.

(qemu) info mac-table vnet0
vnet0:
 \ promisc: on
 \ allmulti: off
 \ alluni: off
 \ nomulti: off
 \ nouni: off
 \ nobcast: off
 \ multi_overflow: off
 \ uni_overflow: off
 \ multicast:
    01:00:5e:00:00:01
    33:33:00:00:00:01
    33:33:ff:12:34:56

Signed-off-by: Amos Kong <akong@redhat.com>