NIC_RX_FILTER_CHANGED is sent by qemu any time a NIC driver in the
guest modified the NIC's RX Filter (for example, if the MAC address of
the NIC is changed by the guest).

This patch doesn't do anything useful with that event; it just sets up
all the plumbing to get news of the event into a worker thread with
all proper locking/reference counting, and provide an easy place to
add in desired functionality.

See src/qemu/EVENTHANDLERS.txt for information/instructions on adding
a libvirt-internal handler for a qemu event (using
NIC_RX_FILTER_CHANGED as an example).